The answer to this can be found in Act II, Scene 1.  This is the famous scene where Macbeth has a vision or hallucination of a bloody dagger.

This means that Macbeth is feeling guilty already about what he is about to do.  He is starting to feel that this is something that he really should not do.

But then his wife rings the bell (the signal to go and kill King Duncan) and he goes ahead and kills the king.

His last thoughts are something like "I am starting to not really want to do this.  But there's the bell.  It's telling me to go and I'd better do it."