The first thing to remember about forgiveness is that it is always a choice. If one chooses to forgive, one may certainly do so; however, it is also possible for someone to withhold forgiveness for a variety of reasons. In this case, you are asking if readers can forgive Lennie for killing Curley's wife in Of Mice and Men by John Steinbeck, and the answer must be yes.
We know from almost the first moment we meet Lennie that he is mentally challenged, and that is the first and primary reason to forgive him. He is childlike in his actions as well as his thinking, and he clearly does not understand the consequences of most of his actions. George has to remind him not to speak, where to meet if they get separated, and what happens if he pets his animals too hard. Lennie is not able to think or reason for himself, so murder is just another word to him. It carries no meaning and no consequences.
We also know that Lennie has killed other things by petting them too hard and does not even really understand what he has done. He kills both a mouse and a puppy just by petting them too roughly (both because of his size and his mental disability), so we should not be surprised when the same kind of thing happens to a human being. He no more understands how he killed Curley's wife than he understood how he killed the mouse or the puppy. Slim describes Lennie this way:
"[H]e's jes' like a kid. There ain't no more harm in him than a kid neither, except he's so strong."
While it is not generally a good thing to blame the victim and clearly she did not intend for this to happen, Curley's wife got herself in this predicament by being so needy. This does not put the blame for her murder on her, but it does allow us to forgive Lennie for it. She is the needy one who craves attention from any male, including the giant, childlike Lennie. If she had been more perceptive and less needy, she would never have been in this situation. She practically forces Lennie to touch her smooth, shiny hair, and what happens next is beyond the control of either of them.
Finally, Lennie obviously has no intention of killing Curley's wife. The incident happens in a frightening moment for Lennie, and he simply reacts. There is no malice and no intent to kill. While it is an awful thing, we can forgive Lennie because we understand that this was a reaction rather than an action.
Lennie was in a panic. His face was contorted. She screamed then, and Lennie's other hand closed over her mouth and nose.
Some readers may not choose to forgive Lennie for murder because, of course, murder is a heinous act; however, he certainly can be forgiven based on these compelling arguments.
I guess we are expected to forgive Lennie for killing Curley's wife, since that seems to be what the author John Steinbeck expected from his reader. There are several important factors to consider. First, Lennie did not approach the girl; she approached him. He was staying by himself in the barn, playing with his puppy,l trying his best to stay out of trouble. Not only did Curley's wife sit beside him, and then move closer, but she actually invited him to stroke her hair. Lennie didn't know what he was doing, and the girl didn't seem to know what she was doing either. Lennie did not intend to kill her. He might have been convicted of murder if he had been arrested, and he might have been lynched for committing murder if the mob of angry men had caught him; but what the reader, who is the only observer of what happened, sees is that Lennie killed her accidentally while he was trying to stop her from screaming for help.
I think we can forgive all the people in Steinbeck's story. We can forgive George for killing Lennie and forgive Lennie for killing Curley's wife. We can forgive her for being so cruel to poor, lonely Crooks, and we can forgive Crooks for tormenting Lennie by suggesting that George intends to abandon him. We can forgive Carlson for wanting to shoot Candy's dog. We can even forgive Curley, who was tormented by an inferiority complex because of his small size.
What made John Steinbeck one of the best American fiction writers was that he had compassion for people, and especially for little people. We can see this quality in Theodore Dreiser and William Faulkner as well.
The child-like Lennie does not kill her because he is angry or aggressive; it is an inadvertent action that was the result of panic. Lennie does not have the capacity to analyze his actions to consider possible future outcomes. He lives in the present; the past is quickly forgotten, the future is not considered. Lennie cannot understand that he has caused pain, or, in this case, loss of life, because he does not intend it. Rather, he blames the victim for being too small and weak:
Lennie looked sadly up at him. "They was so little," he said, apologetically. "I'd pet 'em, and pretty soon they bit my fingers and I pinched their heads a little and then they was dead--because they were so little."
This immature attitude defines Lennie's character; he intends no harm, but causes it. Lennie lacks the capacity to learn from his experiences because they leave little to no impression upon his mind because of his intellectual limitations.
Whether or not one chooses to forgive Lennie or not requires that one beg the question of who,specifically, is at fault. There are several people and situations that contribute to the tragic culmination of the story. If one chooses not to forgive Lennie, it will never hurt him. It might, however, hurt the indivdual who cannot forgive because rancor makes one susceptible to bitterness which is the blight of the human heart.