Space Marines that completely and utterly ignore anything that isn't a threat for them.

Marching through the mob of Chaos cultists, not even noticing the rain of stub bullets bouncing off their armor, the crazy dude clinging on them and trying to pry their helmets off like air, killing off that one Khornate berserker and the slightly bigger cultist that's leading the whole bunch, then sitting down having tea and discussing strategy while the rest of the cultists desperately try to hurt them.

If one of them brings in a rocket launcher with krak missiles, or something else that could actually hurt a Marine, he'll get nonchalantly shot without the Marine even looking at his direction.

Hmm. Or maybe they'd be better as a Chaos warband of Khorne?