Impeachment only removes him from office. Unclear what would happen if he then ordered the FBI and military to arrest and imprison all Senators and just stayed put. Guess it would be an "official act."
What if he’s impeached for something by the House and starts ordering Secret Service snipers (whom he preemptively pardons) to assassinate Senators who say they’ll vote against him at the Senate trial? What does anyone do about that?