We know pretty well where Putin will be on May 9: He will be the guest of honor at the Moscow victory day parade. As president, he is the commander in chief of the Russian armed forces.
If the Ukrainian forces got heir hands on, say, a conventional cruise missile or some other means to kill Putin, would that be legal warfare or would that be a war crime or similar violation of the laws of war?
As I understand it, the general legal opinion seems to be that acts of war are not restricted to current theaters of engagement (which would all be in Ukraine and Russian border regions as it pertains to the current war between Russia and Ukraine). That is how the U.S. justifies its drone killings.
For the sake of the question we'll also assume that all due care has been taken to minimize civilian casualties by using a conventional warhead of the minimal strength sufficient to kill Putin given the precision of the weapon and reconnaissance.