How can someone be both a usurper and traitor? Traitor doesn't know their team, usurper needs the GF killed to win, correct?
What's your claim again, Jeff?
First of all, there's no reason afaik that these two roles actively contradict each other - at least based on their mafiascum descriptions.
Usurper needs to survive longer than GF. Traitor doesn't know team. The not knowing team thing is kinda random in the grand scheme of things but not contradictory per se.
Also, at the risk of falling back into setup spec, scum PMs specifically mention:
"You win when
Arthur Petrelli or Angela Petrelli, and all other factions, are dead (or if nothing can prevent the same)."
The or is very interesting here - it certainly implies that there is some weird dynamic going on with regards to these two characters - although I don't have enough theme knowledge (or creativity half-way through the work day) to determine what this dynamic may be.
But given the logical possibility of usurper and traitor working as a combination, and given that it would be bizarre to invent a role that includes traitor that scum wants to get rid of, I'm inclined to believe that role cop claim.
Or at least, mostly correct, as Guts could have tweaked elements of them.