What do we learn about Lennie in his confrontation with Curley

In the fight between Lennie and Curley, we see first-hand that there is a deep and ruthless capacity for violence in the generally docile Lennie. This violence is sometimes casual and inadvertent - as in his accidental killing of the mice in his pockets - and sometimes an explosion of directed rage, as when he crushes Curley's hand. Lennie seems willing to kill to protect the things he loves, whether George or the rabbits or what have you. His violence is child-like - or dog-like: the sudden ferocity of an otherwise affectionate pet. His casual declaration that he will snap the necks of any cats who attempt to kill the rabbits on his fantasy farm is shocking - we know that he means exactly what he says.