Lennie Small is a man whose lack of intellectual capacity is over-compensated by a huge amount of physical strength. Due to his limited abilities, every choice he makes ends up being a dangerous choice. Not only is he extremely powerful in strength, but he can also go through bouts of violence caused mainly by fear. You could compare Lennie to a black bear, only the black bear attacks to defend himself while Lennie does not even know that he is hurting something.
Lennie has a soft spot for animals and all furry things. He is fond of rabbits, particularly, and puppies as well as mice. At the beginning of the novel he had been holding a mouse which dies due to the force of the strokes that Lennie gives it. Similarly, Lennie kills a puppy accidentally while trying to pet it. Shortly thereafter, he kills Curley's wife, also by accident, while trying to stroke her hair gently.
Basically, Lennie does not have any form of skills that are needed to survive in a world that requires a balance of strong and weak; of soft and rough. He cannot help it. He is just too sentimental, too ignorant and, ironically, too strong to deal with basic situations. For this reason, it seems as if Lennie kills everything that he touches. If he does not kill something, he breaks something, or he hurts somebody. It is accidental, but it is also a symbol of his situation as a social misfit.