Does Lennie "accidentally on purpose" kill the mice (and the puppy) so they won't strike against him or run away?
For example, when the mice bit Lennie, he "pinched their heads a little", and the puppy "made like it was gonna bite me...an' I made like I was gonna smack him...an'...an' I done it."
In the novel Of Mice and Men by John Steinbeck, Lennie is a central character in the book. He is a very large man physically, but is very simple mentally. George is essentially his caretaker. The mice and the puppy are both soft things which Lennie loves to touch, but he doesn't understand his own strength. When the mice or puppy do animal things like bite when they feel threatened, Lennie reacts by punishing them. Because he doesn't know how hard he actually hits them or pinches them or what they can withstand, he kills them. He never does it on purpose, and truly doesn't understand what he has done wrong; all he really gets is that they are no longer alive for him to pet. Lennie does want to keep them, but really just wants them as soft pets, not something he wants to hurt let alone kill.