I agree that when he was created his reasons for hating Peter/Spider-man was weak. But,if I'm not mistaken,in the comics didn't Lex Luthor start hating Superman because he lost all his hair? lol I think over the years,Venom/Brock's motivations have become alot better and so has the character. For example,in shows like Spectacular Spider-man and even(in bits)in Spider-man 3. But I also think you could say that Eddie was never of sound mind to begin with,and the symbiote just made him more nuts and evil. It gave Eddie great power,but it also tortured him in many ways. Not everyone who commits evil has a legitimate reason for what they do,or at least a sane one. They are driven by forces or reasons that are beyond understanding. You can't make sense out of insanity. That's how I look at Eddie Brock. A man who suffered from depression,self hatred and had a huge chip on his shoulder. When you combine that personality with an evil symbiote then that makes for a truly twisted and monsterous monster.