Meet us at the OK Corral… and bring a gun, because you might end up having to shoot a dog. In Of Mice and Men, justice happens the cowboy way. The ranch operates by its own set of rules, without a higher order dictated by ethics, legal precedent, pity, or even common sense. Slim, the local ranch man of wisdom, hands down decisions, and the people around him accept his word as what's best, even if it's not always easy. Sometimes it means you have to get beat up because you had it coming, and sometimes you have to kill your best friend because it's the right thing to do.
Lennie has no grasp of the consequences of his actions, so justice has no meaning for him.
Steinbeck suggests that mental disability shouldn't make someone immune to justice.