Nothing other than common decency stops a president from executing all rivals of their party, pardoning all those involved, then resigning from the office, turning it over to the VP, before Congress could impeach. Now if the only remaining members of Congress belong to the President’s party, the odds of impeachment diminish significantly. In any case, only one person - the President - could ever be held responsible.

But anyone dumb enough to try this would start Civil War 2.0.