The day of Harry's death was a pivotal one in two ways. The first was that Tom realized that he could be motivated on behalf of others-because he was determined to change history so that Harry lived. The second is that he realized Dumbledore understood, at least to some degree, that Tom was rash and not bound by the moral bounds of those on the "good" side against Voldemort.
He made sure, after, that nothing made Dumbledore pay attention to him again. He did everything to impress his professors, followed all the rules (mostly), and didn't so much as get points taken away from Ravenclaw. He used his time-turner to attend all of his classes, got 8 hours of sleep a night, became a perfect prefect in fifth year, and spent all of his Hogsmeade weekends with Ron.
Dumbledore had no reason to suspect him.
However, every chance he got, he'd twist the time turner, close the curtains of his bed, and plot.
The fury he felt at Harry's death never got any easier. The rise of the Dark Lady's regime, the escape of her protege Bellatrix Lestrange and her cohorts, and the Ministry sticking their head in the sand regarding the revival of the war only reinforced his cold, simmering anger. He channeled his rage (and, admittedly, fear) into thinking of a way to save Harry and in getting petty revenge on his blood purist classmates. Watching Malfoy get third degree burns from a seemingly accidental mix up in potions was satisfying but not enough to quench his need for revenge.
Harry was his. Harry and Ron had saved him from the troll in first year. He owed them loyalty. Tom didn't feel remorse or the need to interact with others as friends and at first it was purely because Tom's own ideas of how one should and should not act said that putting their lives on the line for him meant he owed them a debt.
Eventually, however, he began to enjoy their company. It became less about debt and more that they were his—his friends that owed him time, attention, and love. He may not have understood it but he could see it was very emotionally powerful for them and respected it. They felt he was theirs too—the third member of their "best friend" group. Whether or not they thought of it in those terms didn't matter. They held him in high regard and, thus, he did them as well.
Killing Harry was personal and Lady Voldemort would regret the day she ever heard of Harry Potter's existence. She was going to pay for this theft in every possible way that he could work to destroy her and everything she stood for. Once he averted Harry's death, he would turn his single-minded goal to destroying Voldemort.
Meanwhile, Ron spent a lot of time with him, both socially and when Tom was working on their time travel issue. They weren't in the same house, but they would meet up in the recently found Come-And-Go Room and set it up to be a study space for him but a place to relax in for Ron. Tom didn't mind because, occasionally, Ron was intelligent and useful. While the advanced research went over Ron's head, he did whatever he could to aid him. Tom would assign him to summarize articles and books so that Tom didn't have to read all of them as well as use his familial connections to get in contact with scholars on Tom's behalf. Ron groaned about it but always did it, noting when he wasn't sure if he understood something correctly so Tom could look at it himself. Not to mention that Ron would make funny commentary in his notes that sometimes made Tom chuckle.
They bonded, as much as Tom could bond with anyone, over their desire to see the fall of Lady Voldemort's cause and to prevent Harry's death. Tom never got over his anger at the death of their mutual friend while Ron never got over his grief. Ron began distancing himself from most people, clearly going through some complicated grief. Tom knew something was amiss but, quite frankly, didn't know how to help. So he let Ron sit with him in silence when Ron seemed to sink into one of his more somber moods. Ron and Tom didn't have a lot in common, but they mutually enjoyed chess and puzzles. Ron also, after Tom suggested they begin looking into politics and strategy on how to fight and rebel against Voldemort, would have insightful discussions with Tom that not only proved Ron was no idiot but also would explain some of the facets of emotions and empathy that went over Tom's head on occasion.
It took the whole summer after fourth year for Tom and Ron to agree on what they would do to save Harry—it was clear that the best thing for Harry would be to arm James and Lily Potter and attempt to kill Lady Voldemort with a surprise ambush on the night of her attack on the Potters. Going back only to where Harry died would be too dangerous with the amount of Death Eaters that were called to the graveyard, according to testimony from Snape, and the lack of information about what happened between Harry and Cedric arriving at the graveyard and the Death Eaters being summoned meant that there were too many variables to risk it. The night of the Potter family murder was a well-documented and recorded event—not to mention that Lady Voldemort went alone to face the Potters. Her against a wandless witch and wizard was one thing but facing four armed people in a surprise attack was quite another. The element of surprise would be their best weapon.
Tom just had two nearly insurmountable hurdles—going back in time to the night of the murder with only the current research on time travel, which was heavily censored by the Ministry, and their wits. The second, which Tom honestly believed impossible, would be getting back to the future. It took Tom a long time convince Ron that if they wanted Harry alive they had to face they would most likely be stuck in the past.
"If you can't handle it I will leave you here," said Tom flatly over their conjured study table in the Come-and-Go Room.
Tom extensively studied every day for hours beginning the summer right after Harry's death and through fifth and into sixth year. He wrote to scholars far and wide about his fascination with the study of time due to his time-turner and received the information he needed when exhibiting enough charm and false flattery. He snuck out of Hogwarts, even skipping classes, lying about his age and identity to meet with researchers. He also convinced his parents that he didn't need funds for studying at university abroad, which parents had saved up for, and could use those funds for magical study. He manipulated his family and his therapist into believing that his intense research was his way of dealing with "complicated grief" regarding Harry's death. They bought whatever rare (and expensive) text he'd politely and respectfully ask his parents to get him as a result.
He eventually got into the study of rituals, especially ancient ones, which actually got him writing with a hidden Death Eater until a seemingly normal letter nearly cursed him. However, apparently nothing about what he was doing was gaining the attention of the Dark Lady herself, much to Tom's relief.
Ritual research inevitably led him to the risky study of the Dark Arts. Not only would being caught ruin his perfect prefect facade but could also lead to his expulsion or, worse, sentence to Azkaban. But for Harry he'd risk it—and he admitted that the rush of exhiliration and power that came with mastering more and more spellwork was thrilling. The twisting turns and odd corners of Knockturn Alley were disgusting, dirty, and dangerous but held so many secrets that Tom spent a lot of his time there during summers. It helped, too, that he charmed and bribed various shops there to get books on topics he needed.
It was in September of his sixth year, right after his birthday, that he completed his project—and scared Ron shitless by waking him up in his Gryffindor room despite being a Ravenclaw.
"Quiet," hissed Tom, covering Ron's mouth when he nearly shouted, though he was unable to hide his mad grin. "Longbottom loses his passwords so often because I keep up with them. Who knows when Bitch-Who-Must-Not-Be-Named may strike and I need to get to you quickly."
"Has there been an attack?" asked Ron quietly, reaching under his pillow for his wand.
"No," said Tom. "I don't have the patience to wait until morning to tell you. Ron, I did it. I can send us back. Come with me so I can show you."
It was more an order than a request but Ron got up immediately and followed Tom to the Come-And-Go Room.
"There's nothing on the floor," said Ron, looking around. Tom and Ron had realized months ago that going back with a time-turner alone was not feasible and would require adjunct rituals to boost its power.
Tom tossed himself on the conjured couch, feeling let down by the lack of dramatic excitement from the clearly half-asleep Ron Weasley. "No, idiot, because the Come-And-Go Room isn't only known by us. Putting any symbols on the floor would be dangerous and could expose us. However, there is a place we can—and will—perform the ritual."
Ron sat down next to Tom. "Okay, go over this again with me."
"Fine, I'll explain it like you're a first year," said Tom with a sneer as he held up the time turner around his neck. "This is essentially the lightest and least dangerous model of a time turner. It can only go back, at maximum, forty-eight back on its own. The further you attempt the more likely it is you'll break it. There are heftier and stronger types of time turners. The best ones, only used by Unspeakables, are only able back only ten years. There is no way you or I could ever access one so our best bet was to amplify the time turner effects with magic, as we've been working on. Runework and alchemy are good auxilliary aides but simply aren't enough. However, ritual can do it—with unstable results. Magic has not always been controlled by wands. Magic was accessed via ancient ceremonies and traditions in looser ways by both Muggles and magical people to try and influence their environment. Ritual work is not an exact science and the most difficult part in crafting a ritual for us to use was specifying time and place. However, I think I know what we need to do."
Tom couldn't sit still anymore. Practically trembling with repressed excitement, he got up and started pacing as he continued to explain the final results to Ron. "We'll be doing the ritual on Halloween. Place will be, to connect it to the Potters, on their grave in Godric's Hollow. The closer our ritual mirrors the time we're trying to access the more likely we are to arrive there. However, I didn't think we had enough force to power the spell. We may not get in the exact year—there's no way I can guarantee that—but it has to be at least fifteen years ago. I told you we'll be sacrificing everything if we go-there's no coming back, and we may be living potentially four or five years before Harry is even born—"
"Wait, you're serious? Bloody hell, you mean we'd just be waiting—"
"Oh, you cowardly little Gryffindor," sneered Tom, stalking over. He was shorter than Ron but he drew back as if Tom were looming over him. "You told me you would do anything to save Harry. From the very beginning I made it clear that there was no coming back and that inevitably we will be adrift in time. I can't believe you are wavering now, just as we finally get somewhere, while my sociopathic arse is ready to the past to save someone. I thought you were supposedly better than me. You've told me you care about Harry, after all, and claim I don't."
Ron bristled at the insults. "I would do anything to save Harry. Being frustrated by the idea of sitting around waiting for years isn't me backing out!"
"Then it's settled," Tom said with finality. "We'll be sneaking out on the thirtieth. Meet me under Harry's visibility cloak near Hagrid's hut at ten and don't wear your uniform. We'll be sneaking into the Hog's Head and using the open floo to go to to Juniper's Tavern in Godric's Hollow. From there we'll work. As we've planned, bring the charmed bags."
Tom and Ron, in preparation for their trip, had began putting away money (mainly Tom on this but Ron had taken a summer job), clothes, and fake paperwork in order to have identities in the past. Tom and Ron—Ron far more reluctant than Tom—had tracked down forgers and criminals that, for a price, would create a whole life for you. Finding unknown and missing people was easier than expected, actually, with the result that Eric Warrington and Joseph Prewett would become their identities.
Ron had gone from being pale to looking like he might be sick. However, he nodded determinedly. "Got it. I'll be there."