It has to be an official act within the scope of the executive branch. So he couldn’t just bring a gun and shot him, however he could direct the justice department to focus on domestic terrorism and cite Trump’s threats for political retribution as a terroristic threat and have him and every other Republican who publicly agreed with him disappeared.
https://youtu.be/-isSZhkAVKA?si=w_5mhFC79DJckXZ9
Better hope it only has to go on the paved streets.