Lennie is like a child, so he doesn't understand that he is doing anything wrong. When he tried to touch the dress of the girl in Weed, it was just because her dress looked soft. He doesn't realize the consequences of his own behavior, and it caused George to lose his job and be on the run.
When Lennie kills Curley's wife, he doesn't really understand the implications of that. In his mind, he was just trying to get her to be quiet so that he won't get in trouble. This is the worst thing Lennie has done, and because he didn't do intentionally, there is an innoncence behind his actions. However, that means George is the one really stuck with the consequences of Lennie's behavior.