There is a Minchas Chinuch that asks why you aren’t allowed to desert the army for Pikuach Nefesh (and why one is obligated to join in the first place). He says that some Mitzvos that inherently require putting oneself in danger cannot be avoided due to Pikuach Nefesh.
A Jewish monarch that was appointed Al Pi Navi would need another Navi to get rid of. I’m trying to think of a source but I am not sure how one would Halachically get rid of a Jewish, let alone non-Jewish, leader that was elected or self-appointed.