He understood the facts of what he had done, but my Hawke didn't get the impression that he really understood the moral implications. He showed an acceptance of justice, but didn't appear remorseful or capable of any future mercy - which made my Hawke think Vengeance had corrupted him so thoroughly there was no going back.
Not trying to argue objective truths about Anders, just saying how my Hawke perceived the situation. 
Oh, I'm not trying to say your decision was wrong! I just had no idea where you were getting this idea that Anders didn't understand what he was doing thing from, so I was utterly confused.
But yeah, he wasn't sorry for what he had done. He was sorry for the innocent people caught in the middle, but he believed in what he was doing. It wasn't a sunshine and daisies happy decision, but something needed to happen to usher in change and he decided to be the one to take that first, erasable step.
One man's terrorist is another man's freedom fighter and all that, I suppose...