Well he can from Russia by not murdering Putin. If they killed Putin his own life would be forfeit and I suspect that's a big reason Russia doesn't kill him. That and they don't want to martyr him when he's going to grow increasingly unpopular. But it leaves a kind of line uncrossed to not give justification to the Ukrainians doing that because at that point Russia not only nukes Ukraine but they probably have to nuke some of their supporters, perhaps Rammstein airbase in Germany, or some big NATO base in Poland or the UK or some US proxy force that allows them to express their displeasure and inflict pain and dissuasion from ever doing that again without directly hitting the US in a way that forces them to hit Russia back directly.

From the nazis in his own regime is another matter but he does seem to have consolidated power during the course of the war by removing rivals and adversaries so he may be quite a bit safer than he was at the start of the SMO. I think honestly if Trump could get rid of him super easily with a guy waiting in the wings he wouldn't mind it because of the personal disrespect and lack of groveling and ass-kissing he displayed to him.