You think the war ends if they kill Putin? Medvedev would very likely nuke Kiev 30 minutes later.
Trump would almost certainly not intervene with nukes to protect non-US soil if Russia launched first, but there are plenty of European countries with nukes. Anyone in charge of Russia would have to accept the possibility that a first strike would leave them with a bad time.
Now the escalation ball's on our court. Are you willing to die for Latvia?
(And Medvedev does appear to be that bonkers)