Weeeeell, I don't really think he's all that nasty. He's far from a good guy (pun not intended) of course, but at the same time he has his reasons for being the way he is -- and, most importantly, he wants to change. He truly did love Marian, and the shy, eager to please, and stuttering mess he became around her really just got to me, what can I say.

Personally, I find Robin to be more nasty than Guy. Not in comparison of the bad things they've done, but rather in the fact that no one (not even Robin himself) seems to acknowledge what an utterly cruel asshole he can be. Guy does bad things, but he also despises himself for it and wants to change.

...Sorry for going on and on ^_^;