However, if I were Putin I would not threaten Z-man with the loss of his cities. I would threaten him with loss of something a bit more ... personal.
Wouldn't make any difference if Putin assassinated Zelensky. Unlike Russia, Zelensky is not the generalissimo and President-for-life upon which the whole system hangs. Ukraine would just pick someone else and the fight would go on.
Plutonium 239 has a half-life of over 24,000 years.