I agree with clane on this one. Hamlet loved his mother, which made what he saw as her sin all the more unbearable. It took a great deal for him to finally say those horrible things to her that he said in her bedchamber, so I just don't think it would have happened.
HOWEVER, isn't that what "crimes of passion" are? Crimes committed in the heat of a moment, oftentimes by people you would never think could even commit such horrific crimes?
I think I'm about to contradict myself, so I'll just leave it at that! :)
No I don't think so. He barely had the stomach to carry out the one murder plot and he had overwhelming motive to do that not to mention a direct request from the ghost of his father. I don't think he could have done that to his own mother, he seemed to care for her greatly, otherwise he would not have been so angry at her choice and timing of her new marriage. It's true that she was fearful for her life in Act 3 Scene 4 and she asks Hamlet if he plans to murder her, but I don't think he would have ever intended to do that, he wanted to make her understand him, not to kill her even if he wasn't forbade to do so.