Somewhere in the castle Macbeth sits alone, contemplating the murder of King Duncan. And it gets a little complicated. See, if it were simply a matter of killing the king and then moving on without consequences, it wouldn't be a big issue.
The problem is what happens afterward —the whole, being damned to hell thing. It's even worse, because murdering Duncan in Macbeth's own home would be a serious violation of hospitality. He's supposed to protect the king, not murder him. Plus, Duncan is a pretty good king (if not a bit "meek") and heaven is bound to frown upon murdering such a decent fellow.
In then end, Macbeth decides that it's probably not a good idea to commit murder. He has no justifiable cause to kill the king and he admits that he's merely ambitious.
And then Lady Macbeth enters.She gives him a good tongue-lashing, questions his manhood, and lays out the plan to get Duncan's guards drunk and frame them for the murder.
If Macbeth can't keep his vow, she says, then he isn't a man.
Macbeth is a little turned on by this show of strength, and he finally resolves to go through with the murder.