So, I never typed this up. Mind you, this is all first draft quality (and I typed it up while playing TF2), so there might be typos. Regardless. Here!
The "accident" occurred when he was about five-years-old. He'd always had a bad and unnatural fascination with fire. By that age, he'd already started quite a few accidental grass fires and never gained a natural fear of it. There was a night when his parents went to bed and he was left to his own devices. He had "borrowed" his father's lighter and was playing with it in his room. It burnt his fingertips and he dropped it while it was still lit,. The fire spread quickly and uncontrollably. Michael, as he was known then, was the only one too survive, but was badly burned as a result. He nearly died from the severity and was left permanently scarred.
He had no family to take him in; they knew of his pyromania and didn't want to risk a repeat of what occurred. He was put into an orphanage where he was quickly outcast for how he looked. Michael hated the other kids and frequently acted out in violent ways from their bullying. The one person that was nice to him was the kitchen cook who taught young Michael how to cook. He enjoyed it and kept it up, but it didn't stop his firestarting.
He was in and out of foster care for his uncontrollable and sometimes violent behaviour. He became known as unadoptable and was reaching the age where he would no longer be under state care.
At 16, he was pretty much put out on his own. He was homeless for awhile before he found a carnival seeking workers. Since Pyro had talent with fire and already looked the part of a carnie, he figured it wouldn't be too bad of an idea. It wasn't and it worked out to his advantage. He fit in with the rest of the sideshow freaks surprisingly well, becoming friends with people for the first time since his accident.
He thrived there as the firebreather as well as becoming the cook for the sideshow. He became less known as Michael and more known as Pyro. He designed and built his own flamethrower during that time, perfecting it as he went along. He spent somewhere around ten years as in that "family" of freaks. When he was offered a job with RED, it was a difficult choice, but he decided indulging his violent urges was better than carnie work.