I don't think there was enough of a buildup if that was supposed to be Terra saying he'd be the bad guy if that's what it took. He's bitter and angry, but I'm not sure that he'd go so far as to say he's sick of being the hero -- that's just too far from Terra's personality to happen so quickly. I can see him getting so wrapped up in whatever that he just doesn't care about anything anymore, especially considering the way Terra said that line, but he'd still probably want to be a good person.
Xemnas is something different entirely -- whatever Terra meant by that line, I don't think it meant the kinds of things he did to Roxas and Xion. =/
Saying he doesn't want to be a hero anymore might be a stretch actually...
But everything adds up honestly. There are way too many hints that Xemnas is in fact emotionless Terra.
Oh boo hoo, Xemnas ruined Roxas and Xion's friendship. These two are the ones who turned against him in the first place. Sure what he did to them was cruel, but not unexpected or hard to understand.
Anything cruel Xemnas did was only natural for someone with no emotions and thus no guilt.