Мафия 51. Pandorum

I would execute Arka; she is silent, secretive, and useless. Plus, I've played Mafia with her several times and tend to agree that she really has a certain paranoia. This hasn't been observed in Ment.
By the way, there's also Kosha. He's also a perpetual paranoid and suspiciously silent player.

Judging by your "failure" (if you can call it that) with Arka, about her not being online, and you just removed Ment, easily clearing him of suspicion, either you have a super-sense or the ability to read people, or, like Ment, you are in cahoots.