The conspirators were hanged for 'high treason'.
He is guilty of 'treason'.
(..) you’re under arrest, Chancellor. Are you threatening me, Master Jedi? The senate will decide your fate. - I the senate. - Not yet. It’s treason then. - - You are under arrest, my lord. Anakin, I told you it would come to this. I was right. (..)2005 Star Wars: Episode III - Revenge of the Sith
(..) The council wants you to report on all the chancellor’s dealings. They want to know what he’s up to. They want me to spy on the chancellor? - But that’s treason. - We are at war, Anakin. Why didn’t the council give me this assignment when we were in session? This assignment is not to be on record. (..)2005 Star Wars: Episode III - Revenge of the Sith