Declare him to be a Clear and Present Danger to the Union. Now it's an official act to safeguard against the danger. So take official action, then wait and see if anybody prosecutes after the fact to prove it wasn't an official act.

Well we used to straight up kill traitors instead of letting them off and letting them try running for an office that they once tried to steal multiple times through various nefarious ways.

Killing a traitor sure sounds like an official act.