They don't have that much power. It takes a lot (and a lack of sanity) for someone to "kill a bunch of people on their way out".

The most they can do is something like Jan 6th with more competence. Something like that requires organization, and the FBI is pretty good at infiltrating organizations who are trying to grow enough to make a difference. January 6th wasn't stopped primarily because it was done by the President, and the office (for better or worse) gets more leeway with laws.