The window for that has closed. The national mood is firmly anti Russian. Certainly Zelenskyy can be killed, but Ukraine isn't a Russian-like autocracy or dictatorship - there is no metaphorical head of the snake to cut off. Ukraine has a functioning democratic government.
Ruslan Stefanchuk, the chairman of the
Verkhovna Rada, Ukraine's parliamentary body, and a close ally of Zelenskyy would take office if Zelenskyy was unable to perform his duties, with limited powers as "acting president" until a new election is conducted. If both these guys die, then Prime Minister
Denys Shmyhal will administer the state until the President and chairman are replaced. There is an established succession process - no Russian puppet is getting in again - 2014's expulsion of Putin's man Viktor Yanukovych saw to that.