Can I say She instigated the murder?

To "instigate" something is to push it forward but not necessarily do it yourself. For example, "When the teacher announced that he was going to flunk the entire cl, he instigated a riot." (It was the students who actually did the rioting, but the teacher "caused" it by his actions.) In your case, Lady Macbeth took several actions and said several things which pushed her husband to commit murder. Your use of "instigate" strikes me as correct. (Pay attention to the way it is spelled.)