Why stop him from killing anyone, if you are going to let him live to keep doing it? It's the same thing. He knows he'll keep killing, over and over. Why stop him while he's killing, it's effectively the same thing.An important point about the Star Wars universe is it is a metaphysical universe. Whatever you believe in the real universe, good and evil, fate and destiny are very real things in Star Wars. And striking someone down in cold blood is definitely a dark side (i.e. evil) action, and leads directly to Anakin's fall. Anakin strikes down Dooku, Mace Windu tries to strike down Sidious.
Grey pragmatism may work in real life, but in the Star Wars universe no good can come of it.