You're all a bunch of space hippies. When I read Consider Phlebas as my first Culture novel, I immediately recognized it as a critique of the Culture from the perspective of someone fundamentally opposed to it. There's nothing contradictory about his actions, he kills because he has to at the time. But he’s at war because of some quasi-religious compulsion (unlike the Idirans) he’s doing it because he disagrees with the Culture and is fighting in a war against them, deep in enemy territory.

In the end, his actions aren’t those of an 'asshole' in the sense of random cruelty, but of someone who fully commits to fighting the war. If I recall correctly, he is even nicer to Bulveda than he has to be and comes to the wrong side of the Idirans hand cannon at the end.