A/N: So here's my attempt at Billy's origin story
When Billy went into the Super League of Heroes headquarters, they had taken one look and immediately laughed. He couldn't control that odd double-blink habit whenever he was nervous, and avoided the interviewer's eyes the entire process. It didn't help that he could hear the man's patronizing undertone. So he wasn't that strong – at all, really – and he didn't have the bravest disposition. But he was smart, and could invent gadgets that would take down even Bad Horse. Of course, the final nail in the coffin was when Billy had started talking about his ideas for social change. That was his greatest mistakes – heroes didn't want to change the world – they wanted to preserve it. They were the dumbest of the dumb, brainless sheep that would flock around their corrupt ideals and stop necessary change – and there was no arguing that reform was needed.
The world wasn't supposed to be a place where dumb brute strength of heroes was valued over a quick wit and quicker mind. Humankind had always thought themselves better than animals due to their superior intellect – so why didn't they value that, instead of something any common animal could do? Reform was needed, and like everything else sacrifices had to be made. He couldn't change the basic equation of his country (power = strength) by talking. No, action was needed. And who else could so deeply shock a nation – no, a world - more than a supervillan prevailing because of his brains? He would show them that only intelligence could bring him down – intelligence that they would then strive for and try to earn for survival.
Of course, in order for the plan the work he would first have to become a supervillan, which would raise complications and a lot more variables than he liked. Suppose he went completely evil, and lost the plan? Suppose he actually was defeated by a dumb brutish superhero? Billy scribbled on his chalkboard, creating equations and possibilities in his safe world of math. He re-did the calculations, double-checked his work. He created variables for every conceivable event, and made his decision. Yes, there were a few possible bad outcomes, but only if a metaphorical perfect storm brewed in just the right conditions. Thus, Dr. Horrible was created and Billy began his struggle for social change – after all, what were the consequences of his perfect storm actually happening?