He wanted to help people, but the staff just couldn't handle having a 14 year old colleague. The disrespect was never overt, not at first, but by the time Dr. Howser's late-teen hormones had really kicked in, his disdain for his so-called colleagues' inability to treat him as a fellow practitioner had maligned into a deep hatred of inferior, inflexible, unenlightened minds.
Thus, with his residency complete, Dr. Howser disappeared, and Dr. Horrible became manifest.