I searched Spiderman's enemy page for a good 30 minutes and totally think "Crazy Eight" is the best villain origin and motivation. "Best origin" meaning that he has none at all. I hadn't heard of him before today, but this is a part of the appeal. He is a normal person that wields eightball shaped weaponry. This is good not just because its funny to think there is no motivation for an eight ball wielding villain, but because its a simple, clean motivation and origin. Anyway can look at Carnage or Doc Ock or Green Goblin and say "Why do they care about staying in New York?" or "They shouldn't ret-con this relationship?" Ultimately what I am saying is, you cannot question and poke holes in nothing and sometimes no reason is the scarier than anything you can come up with. Crazy Eight is just a fun character to prove this point.