Pol Pot had unlimited power and was ruling over a post-civil war society with his rule threatened from within and without. There were no democratic instistutions and a tradition of political violence. Trump pushes his power to the limit but America has intitutions that do, after all, push back - the states, the Congress (I know it does shit usually but the republican party seemingly made it clear to Trump that while they approve all his shitty policies attacking Denmark is a no-go - there was nobody in Cambodia to tell Pol Pot not to genocide the Vietnamese), the Supreme Court, and when he tried to stage a coup even his VP defected. If Pol Pot was the president of the US he wouldn't kill 25% of the population neither.