Can you tell me some reasons why Lennie in Of Mice and Men shouldn't be punished?
There are reasons for and against Lennie being punished (and even an argument to be made that Lennie's death is not best seen strictly as a punishment).
George offers one of the best reasons not to punish Lennie when he, along with the others, comes to the barn where Curley's wife is found dead.
"The poor bastard's nuts. Don't shoot 'im. He di'n't know what he was doin'."
Here we see a suggestion that Lennie should not be held accountable for his own actions because of his mentality. If Lennie is actually "nuts" or at least mentally incompetent in a legal sense, we can use that as one reason to argue that Lennie should not be punished. (Punishment, after all, is intended to apply to those who willingly do wrong. If Lennie does not know that he is doing wrong or if he is incapable of controlling himself, punishing him does not seem to be the best course of action.)
It is possible that Lennie does not "know his own strength," as the saying goes. We get information about his killing of mice and watch him seriously injure Curley. Lennie's strength is commented on repeatedly in the text. If we feel that Lennie applies his strength without understanding the full ramifications of this application, maybe we have another reason to argue against punishing Lennie.
Ultimately, Lennie is killed but we might see this as a mercy killing. George saves Lennie from facing Curley's wrath and potentially being put in jail. The punishment of jail or violence might be understood as being far greater than the quiet death Lennie receives from his companion, George.