Now Null could be lying to take the heat off of a town power role, but in that case he would be sacrificing himself for the better of the town, so it's the exact opposite of Cult behavior. Given the Cult's strategy, a vanilla Cultist trying to take a blow for the Cult Leader would be a very bad idea, and one they probably would not take.
Null could be Cult, but his behavior so far indicates the opposite of that. I wouldn't advise anyone to vote for him.
So back to the drawing board.
The two people who died were the most quiet, so that doesn't give us much to work with. However, power roles tend to be quiet while still being involved in the game. Even if what one says is helpful, talking much makes one bigger in the conscience of people when deciding their lynch votes, so for power rules it is usually wise to lie low. Many vanilla townies would feel a lot more comfortable speaking up because it would be better for them to risk lynching than a town power role. Now a power role isn't necessarily a Cult member, but the Cultists are by definition power roles, so targeting them narrows it down some.
Caoimhim, Grek, Null, and Valmark have spoken too much to be power rules. AvatarVecna and the Outsider are getting auto-lynched anyway, so that's not going to help much. I guess that leaves us with Elenna and Book Wombat then as the most likely to be Cult Leader? I will go with Book Wombat, as Caoimhim has already done. At this point I know more of who I don't want to vote for than who I do want to. Anyone have any better ideas?