Lennie kills his puppy purely by accident, when he strokes it too hard. This makes him utterly miserable – not just because he has killed it, but also because now he fears that George won’t led him tend the rabbits on their dream farm, as rabbits are similarly soft and vulnerable little creatures.
The death of the puppy at Lennie’s hands recalls the incident at the very start of the novel when he inadvertently killed a mouse in the same way, and anticipates his equally accidental killing of Curley’s wife just a short time later. He manages to break her neck when, at her invitation, he strokes her hair.
We see therefore how Lennie’s physical strength is such that he can easily kill human beings as well as small animals without realizing it. This is what makes him so dangerous – a quality which contrasts quite startlingly with his actual goodness of heart, and his sincere affection for his mentor George. This is why George sticks with him through thick and thin. He needs Lennie as much as Lennie needs him. And Slim, who understands other people better than anyone else, pays tribute to Lennie’s goodness, his complete and utter lack of meanness which contrasts with so many of his fellow-men:
‘He’s a nice fella,’ said Slim. ‘Guy don’t need no sense to be a nice fella. Seems to me sometimes it jus’ works the other way around. Take a real smart guy and he ain’t hardly ever a nice fella.’(chapter 3)
It is just Lennie’s tragedy, and George’s tragedy, that Lennie lacks the mental ability to temper his great physical strength; and most other people are neither able nor willing to understand him. George strives to protect him as far as possible, but in the end, it isn’t enough.