What right will we have to criticize Putin if he kills Ukrainian president Zelenskyy?

From a legal standpoint, the US has illegally killed the leader of Iran in an illegal war which our leaders are not even criticizing. They are just accepting that the US takes out a leader they do not like, thereby destabilizing a nation of 93 million people.

International law does not allow this, but if we do not uphold international law, what boundaries do we have left to limit dictators like Putin?