He doesn’t trust Lucifer it would seem. If Hercule was immune Leon would try to vote out Lucifer and then be guaranteed F2. Meanwhile Lucifer’s confessional said if Poirot was immune, Lucifer would be voting Leon. So they’re not really as close as it had seemed, which gives Rust slightly better odds to make the end than you might think. But the final vote off is different than the rest, since it’s only about who you want to face in front of the jury, so who knows.
