What is your point? No one argued that you wouldn't kill those who have turned...
There is a big difference in character if someone actually thinks through what can and can't be done as opposed to "lets kill them" and fucking leeeroy the city.
He didn't give a moment of thought at all. <- that's what makes him bad and evil.
Same as if a vet has to put down an animal no one considers them evil or a bad person, except maybe a few. But if you take your animal in and the vet instantly say "I have to kill the pet" then fucking yeah you should be scared of that person. Especially when he doesn't give any sense of sympathy or empathy along with it. Which is what Arthas have never shown to have. He has 0 of it, which sort of make him like a psychopath.