I'm not sure we can call him a psycopath...sure, he manipulates persons, lies (although it's not pathological, just because of his situation), and uses charm and possibly violence to control others to satisfy his own wishes but I don't see him having an antisocial personality disorder, he can socialize and empathize with others (cf. the student council) and he clearly doesn't lack conscience or feelings for others seeing how he worries about Suzaku and his sister and his reaction to his own reminiscence of
He's just a person hell-bent on revenge - and an intelligent one at that.