And he got really, really, really possessed before he killed Giles. Because we don't want anyone to think he's actually responsible for the choices he makes (though we're kind of hazy on which ones were actual choices and which being the universe's puppet. Then again, some people want us to think that he didn't actually do very much that was wrong [except the possessed bits].

So you see, Angel is really mostly blameless... except when he's "right."