This is a good question, because actions are not the only things that speaks of a person's character. In fact, if we only look at a person's actions, we might completely miss the character of a person. What we need to do is to look at a entirety of a person and the context in which an action is performed. This is especially true in Of Mice and Men. Let me give two examples.
First, if we only looked at Lennie's accidental killing of Curley's wife, then we might conclude that Lennie was a murderer. And if we believed in the stories that he attempted to rape a girl before, then we might conclude that Lennie is a menace to society. However, if we look at Lennie more carefully, he is a kind and gentle soul, who does not know how to act in society. Therefore, we would have compassion for him.
Similarly, if we only looked at George's action at the end of the novella in shooting Lennie, then we might think that George is a coward or a bad friend. But if we follow George and walk in his shoes, we would see that what motivated the shooting of Lennie was his loyalty to him. We might not agree with George's actions, but we would not judge him harshly.
These two points show that all actions have a context, and more importantly a single action does not divulge a person's character.