At the age of five, Harry decided he'd had enough.
…Fat lot of good that did him. Coming to a decision didn't particularly matter when he didn't have the authority to make any decisions.
Even then, as a toddler, he knew it would be stupid to run away. Not because he was particularly wise. Rather, he had tried it at the age of three, and had come to the unpleasant realization that adults never took kids seriously, regardless of the situation.
And since all the Dursleys (even Dudley, who was surprisingly the same age as Harry) were at least three times his size, there wasn't much he could do against them physically. (Unpleasantly enough, even that realization had come from personal experience).
But at that same age, something amazing had happened. Something crazy. It was nothing short of a miracle, but there was no way in hell Harry was going to question where it came from. He'd take whatever he could get. (Literally, but he preferred not to think about that.)
He found out that he was different from the Dursleys.
Well, yes. Obviously he was different from the Dursleys. To name a few things, Harry wasn't fat, or stupid, or tormenting an innocent kid. But that wasn't it.
No, what he found out was that he was fundamentally different - he could do things that were impossible for them.
Like make things float just by willing it. Or make them break, or twist, or move.
It was a morbid thought, especially for a five year old, but when Harry realized he could crack Dudley's new television set clean in half (it took some work) seemingly with nothing more than the power of his mind, his next thought was - I can hurt them like they hurt me.
Because if Harry could tear apart a teddy bear without lifting a finger, why couldn't he do the same to, say, Dudley's arm, before he broke Harry's?
But Harry wasn't stupid. If he'd somehow been granted a miracle, he wasn't complaining. And he was definitely not going to just waste it. That was what Dudley would do, and Harry knew he was nothing like Dudley, the stupid pig.
So he used the time spent locked up in the cupboard under the stairs, where he slept. Used it to flick the lights on and off without moving, to unlock and lock the door. (That took some time, because he had to fiddle around with the stuff inside the lock that he couldn't really see, but it was worth it when he finally managed it.) And he used it to make those stupid spiders scuttling out of the corners implode on themselves, and felt grim satisfaction that now he would no longer have to brush the little buggers off his socks.
Harry probably wouldn't ever admit this out loud, but he used to pretend those spiders were Dudley. Or Uncle Vernon. Or even Dudley's stupid gang, who looked to him as the leader only because he was the biggest and stupidest of the lot. So killing those spiders started to take up a lot of Harry's time, especially during long hours spent locked up alone in the cupboard. Figuring out different ways to get rid of them - there were so many. They could explode, or be ripped apart, or just be crushed, or Harry could even make them disappear entirely. When he stumbled upon that last one he was speechless, because all he could think about was the possibility of perfectly, cleanly, getting rid of Dudley forever. Just one second of making him vanish into thin air. No blood, no fight, no evidence…
One thing he knew, though. It was the only thing that stopped him from actually experimenting on Dudley - if the Dursleys found out he could do this, they'd kill him, in the most literal sense. And even with this newfound power, Harry might not be able to stop them.
…Not to mention that he might go to jail, or worse, end up as a test subject for scientists to poke and prod at. That wouldn't be happening if Harry could help it.
Then there was school. Harry worked his absolute hardest to learn everything he could - not because he really wanted to, but out of pride. He learned early on that getting better grades than Dudley would get him punished and accused of cheating - and needless to say, the oaf didn't get very good grades. Ironically, though, that only fueled Harry's desire to continue to do better. Obviously, the Dursleys wanted him to be even stupider than Dudley. Who was already the stupidest person Harry had ever seen.
If they wanted him to be stupider than Dudley, there was no way he was actually going to let that happen.
In the first few years Harry was in school, he tried various ways to still be able to get better grades than Dudley and get away with it. He tried reporting it to the teacher, the principal, anyone. Nobody believed him. All it took was one word from the Dursleys and he'd be instantly accused of cheating and given a zero on the assignment. They touted him as a problem child, an idiot, a delinquent, so who was the school to contradict that image?
Finally he realized that there was a way to avoid being stupider than Dudley, even if he wasn't allowed to get better grades. Starting in his third year, Harry started to study as hard as he could. He'd study extra compared to what he needed. He'd spend hours after school in the library - the Dursleys could never figure out where he went, because he was careful to slip away before Dudley's gang could see him. He probably wouldn't have been able to pull that off without his special powers, which seemed to conveniently strengthen whenever he was in a pinch.
At first the Dursleys were angry, probably because he was missing out on doing their chores at home, but he told them that he was getting into detentions every day with a teacher who hated him unfairly. They loved that. Uncle Vernon sneered that finally there was someone else who saw what a useless, ungrateful brat Harry was and had decided to help set him straight. Harry just hung his head, but inwardly he was filled with triumph.
When it came time for the exam papers, he would purposely calculate how many questions he had to miss in order to ensure a worse grade than Dudley (who averaged 30 marks on each out of 100). Then he would get exactly that number of answers wrong, while mentally noting the right answers in his head. For assignments, he would make sure he understood them but neglect to turn them in.
Harry had never enjoyed studying in the past, but that was when he didn't understand half of what was going on - the teachers, probably tipped off by the Dursleys, would take any and every opportunity to send him out of the classroom and otherwise prevent him from learning. Now, though, he was starting to realize that this stuff was actually interesting - well, some of it, anyway. History could be interesting, but it seemed like a bit of a waste of time when compared to other things - like science. Discovering science was amazing. Everything that went on in the world could be explained - how living things operated, how objects moved in relation to each other, why the sun rose and set, why water acted the way it did.
It was fascinating…or it would have been, but the fact that Harry's special powers didn't seem to follow any of those rules kind of put a damper on things.
Psychology was pretty interesting too, and after reading some books on it he actually started noticing things about how people behaved that he never had before. Like how the Dursleys were suspiciously jumpy about anything at all out of the ordinary, especially around Harry, and how they always displayed all the signs of lying whenever they had to mention his parents. The former made him suspect that they might know something about his "powers", although he had no way to confirm that, and the latter only made him more sure that his parents didn't die in a car crash, and that they probably hadn't been horrible people either. But again, Harry had no way to find out the truth, since obviously the Dursleys weren't telling.
So during the day he did his best to avoid Dudley's gang and then hid out in the library and studied, and when he got home and was inevitably locked in his cupboard he would sit up and practice manipulating things with his mind.
The school library had a lot of books. Way more than Harry had ever expected. Eventually, he started studying things that were ahead of what they were teaching in class. Soon he was devouring any math or science book he could get his hands on. He spent so much time in there that the librarian was starting to get fond of him.
That should have been a warning sign.
Harry should have known it was too good to last.