It's also man!pain affliction... Much like the Tenth Doctor became very unlikeable, utterly swallowed up by his own pain, but still everyone loved him 'just because'. His pain supposedly excuses the way he's a jerk to everyone around him. (I rewatched some of S3 and OMG Ten was such a bastard towards Martha. It's *breathtaking*.)

For him, Angel was like Lestat (the classic vampire/byronic hero, I think)
Which is where the problem lies, because Angel isn't. As you say, he was far more interesting when he was the existentialist guy.