Spectre wasn't born evil, just foul-tempered and mischevious. It didn't matter that everyone hated him as long as he had Rhiannon, a sweet gypsy filly who was pretty much the only pony who believed he had any good in him.
That is until one day her family attacked him (you know, sometimes families just can't accept who their daughter is dating) and he fought back, winning, only he didn't count on hitting Rhiannon with his special life-zappy powers in the process.
No one believed Spectre's "in self-defense" story, however.
So he just decided to finally become what everyone expected him to be. Evil.