I'm not sure if killing Cassius counts as cold. He was locked up with Thorn Manacles that prevent him from using magic, he is badly wounded and doesn't know that Lasciel can teach him a method to allow him to block off pain, Cassius is being held by Mouse but there is nothing to say he couldn't use some form of magic to kill either himself or Butters (he can afterall use enough time to perform his death curse). What happens if Butters can release his manacles, how long can Mouse hold Cassius immobile before he tries something. I think it could be justified as self defence in this case.
Harry feels guilty about it sure, but Harry isn't really a killer and has before this only killed in the heat of battle, killing Cassius and Corpestaker in these ways was bound to make him feel guilty especially given the presence of a fallen angel he is terrified will make him a monster. Is it enough to stain him, probably, but it is nowhere near as bad as killing Slate and on the other side, certainly not the same as justifying killing Marcone.