Was it really even murder? I mean they were essentially in open war against each other (as much as you can be in proper Alethi culture). Sadeas may not have been a regular open combatant, but he threatens to kill and/or destroy Adolin's whole family. That, combined with the treason already committed is enough reason to attack the man. They don't need to be fully armored on a field of battle to make it morally acceptable.
Make no mistake, Sadeas was trying to kill the Kholin's, just in a very drawn out process. Adolin just did it faster.