Why are serial not tuff like career criminals,thugs and gangsters?

I heard on t.v. that your averege bank robber is more violent then your averege serial killer.I agree because Ted Bundy was charming and Jefry Dahmer was outspoken and never got introuble at school or with his parrents.But what Jefry Dahmer did is creapy like cuting heads off puting a acid in his victims head most criminals never did what he did he must have had evil guts do that suff but he is not tuff as most criminals are.Why?He was twiced stabed in prison and then he was murdered.How is he able to kill in this manner and not know how to fight and look like a nice guy?Im not talking about there looks im talking about there personality.