Should all karma 2+ roles be approached as antagonist roles who help drive interaction, conflict and general goings on?
I think they should be people who have goals personal to them, and conflict will arise organically when those goals come into friction with others. They shouldn't be forced to exist as "plot vendors" or purely for the sake of "stirring up" conflict.
No, but karma is awarded to players for doing things like driving RP and understanding game themes enough to reinforce it, so I think it goes hand-in-hand organically enough.
Honestly, the 'big baddy' karma roles are naturally wedged into antagonist roles because the game world forces it pretty handily. It's non-karma roles that are more in need of reward for antagonism. Not direct reward, but through the actual play of it. The enjoyment factor.
Non-karma roles in antagonist roles tend to get dogpiled by the rest of the playerbase with no real boon to doing so. When they -do- manage to both play smart and get lucky, they tend to get ragged on in the OOC discussion of it.