Both of these answers are true. I would add that Macbeth considers he has defiled his mind and given his "immortal jewel" (his soul)to the devil when he killed Duncan. By Act 3 scene 1, he already feels he is beyond redemption. So one could argue that his moral decline begins and ends here, yet guilt at his part in the murder of Banquo seems even more intense. His references to blood have changed here--where once he felt he could dip his hands in the ocean and the ocean would turn red with the immensity of his guilt, now he is actually in the water and the water is blood. The waters are not friendly (blood will have blood) and he is recognizing that he must keep moving (if he is to stay alive). Even if he could return to the place he started from (he can't), he would have to experience so much on the way back that it would cost no more to follow through and finish what he's started. With this statement, he says his moral degeneration is irreversible: he will do whatever needs doing in the service of "his own good" (by which he means to keep the position he has stolen). He has already killed two innocents (four if we include the guards). There is nothing but to continue with acts that have to be done.

tthakkar has a good post explaining the mindset of Macbeth. Literally translated, Macbeth is saying that since he's already killed Duncan, he's in big trouble whether he attempts to reconcile, continues to pursue his goals, or does absolutely nothing. Since he chooses to continuing pursuing his goals (which of course included the killing of Banquo and the Macduff family), he's telling you that he really has no morals left. Anybody who wanted to attempt doing the right thing would fess up to the murder of Duncan, accept any punishment, and do whatever necessary to try and regain some favor from the people of Scotland. Macbeth obviously isn't interested in the "right" thing anymore.