A/N: This chapter was inspired by a line in one of the fics I've read that was a response to the "Don't Fear the Reaper" challenge by Reptilia28. If/when I find which such fic it was, I'll edit this to say.
Harry was just leaving the washroom after his weekly shower (cold, of course; no sense wasting hot water on a 'freak', after all). It was as he was about to head downstairs to begin his chores that it happened.
Decent parents teach their children from a very early age not to do things that could hurt people. Part of this is obviously trying to instill good behavior as soon and as deeply as possible, but another part is that it takes a while for children to begin to understand the consequences of their actions. Until they have grasped both "doing this could hurt someone" and "hurting people is wrong" a blanket prohibition is needed to keep even otherwise good children from unintentionally causing great harm.
Vernon and Petunia Dursley were not decent parents. They weren't decent human beings in general, but it's their parenting in particular that's relevant here.
Dudley Dursley had not been prohibited from doing things that could hurt people. If anything, such behavior had been encouraged by his parents, especially when directed at "the freak". The young boy wasn't a truly evil child; given time and opportunity, he could have grown up into a reasonably good person, appalled by his family's actions towards his cousin. He certainly didn't intend to cause any serious harm, but his only reference for what might happen were cartoons, in which characters might end up looking comically distressed but would always recover by the next scene. Nothing in his limited knowledge and experience told him how badly even simple things could hurt someone.
So it was that Harry Potter, age five, was violently shoved from behind as he stood on the top step of the stairs at #4 Privet Drive. He felt himself falling, tumbling, then his head hit a step at a bad angle and he felt nothing more.
At the top of the stairs, Dudley cackled in amusement at his handiwork. After a minute or so, though, he noticed the Freak just lying there at the bottom of the stairs. "Mum!" he called out, "The Freak is laying about!" He stood watching in gleeful anticipation of the tongue-lashing his worthless cousin was about to receive.
Petunia Dursley stormed from the kitchen. "Boy!" she snapped, "Stop making such a racket and get to work! Of all the useless, ungrateful..." Her tirade trailed off as she saw her nephew sprawled at the bottom of the stairs, unnaturally still, with his head turned at a funny angle.
At first, the emergency personnel bought Petunia's story that her rambunctious nephew had died in an accidental fall. Unfortunately for both her and Vernon, however, one of the responding bobbies felt that something wasn't quite right, and convinced his reluctant captain (an alumnus of Smeltings) to allow him to investigate. "Just to be certain, sir," he'd placated the man. "After all, it's a child that's died; might be trouble if we're not properly thorough about it."
His and his colleagues' return to the Dursley household didn't produce immediate results, but the more they looked over the scene of the boy's death the more that nagging sense of wrongness prodded at them. It took over an hour before one of them realized that it wasn't that there was something there that shouldn't be, but rather that something that should be there wasn't: None of the photos they looked at showed little Harry Potter. Spurred by this realization, they began combing the house with fresh eyes, and began to find a pattern of absences. When they asked about his bedroom, they were pointed to a room filled with broken toys, the lumpy old bed buried under a pile of dusty old playthings. They couldn't find any clothing that clearly belonged to the boy, with everything clearly sized for his much larger cousin. Other than the testimony of the neighbors and the far-too-young body in the morgue, there was nothing to indicate that Harry Potter had ever lived at #4 Privet Drive.
These discrepancies were enough to bring far more focused investigation. A records check showed another pattern of absences; in this case nearly all of the paperwork that would normally be associated with an adoption was missing, as were records of standard vaccinations and health checks, and almost everything related to the boy's parents. When police searched the house more thoroughly, the cot in the boot cupboard was particularly damning.
In the end, Vernon and Petunia Dursley were convicted on charges of abuse and neglect. The Crown Prosecutor didn't bring any charges related to young Harry's death, as in spite of how the child had been treated there was still no proof that his fall had been anything but accidental. When it was discovered just how poorly-behaved and ill-adjusted their son was, Dudley was placed in a home for troubled children in the hopes that he was young enough to be taught how to behave in society.
It took over two years of both growth and therapy for Dudley to realize the truth and horror of what it was he'd done to his cousin. When his anguished sobs had calmed enough for him to explain to the therapist he'd been speaking to, he was assured that it was covered under patient confidentiality. To the man's surprise, the child had fixed him with an intense gaze and demanded that the proper authorities be informed.
Dudley's account was enough to revisit the case of his cousin's death, and his testimony to the coroner and police served as the first step in what would become a lifelong quest to salve his feelings of guilt.
His new understanding of life, and of its tragic fragility, pushed the previously lazy boy to become a far more enthusiastic student as he began to strive towards his goal of becoming a social worker. He would seek to atone for his cousin's senseless death by dedicating his own life to helping other children in bad homes and counseling those heading down the same path he'd been treading. The moment it was legal for him to do so, Dudley changed his surname to Evans.
On the magical side, word of the fate of the Boy-Who-Lived soon spelled the death of Albus Dumbledore's political clout and social standing. He and those closely aligned with him, such as Minerva McGonagall and Arthur Weasley, were soon left as unemployed and unemployable social pariahs. This, coupled with an upswell of anti-Muggle sentiment due the the ultimate cause of the last Potter's misfortune, fed tremendous support and influence to Lucius Malfoy and his allies. By the time several years later that Lord Voldemort returned, he was openly welcomed by Minister of Magic Dolores Umbridge.
Coming as it did after years of ever-escalating discrimination and abuse against all those not part of the blood-purist agenda, this was the spark that ignited the Magical British Civil War. Virtually every non-Dark magical species (and even some that were considered Dark, such as many werewolves) banded together with the oppressed witches, wizards, and squibs to form a resistance. Goblins made war upon wizards for the first time in three hundred years, collapsing the magical side of the nation's economy.
After nearly a decade of bloody guerrilla warfare and even bloodier reprisals by the Ministry, Voldemort was finally defeated in a mass duel against Amelia Bones and a force of former Aurors and disillusioned former members of the Order of the Phoenix, his soul trapped in amber thanks to a modified version of an ancient spell found in the goblins' archives by Hermione Granger. Even with their Dark Lord gone, the Dark Ministry continued on for another three years before finally falling to opposition forces in the deadliest battle of the war. Magical Britain was finally free, but it would be well into the first decade of the twenty-first century before it once again had a stable government and economy, and generations before it could once more be considered relevant on the international stage.
A/N: So yeah, there's my first shot. Dudley becomes a much better person and helps a lot of children, but Magical Britain gets pretty thoroughly devastated and a LOT of innocent people suffer and die. All this because Vernon and Petunia never bothered to tell their son "no" and young children don't think about or understand all the potential consequences of their actions.
Also, I'm intending for at least the first few chapters to focus on things that are theoretically positive. My working titles for the next two chapters are "Kindness" and "Hope" - guess what helps lead to Harry's deaths in those chapters, eh?