All you've done here is reiterate what he said and insult him for thinking it. That's not even remotely persuasive.
The commenter I replied to seemed to unaware how far they had strayed beyond reasonable ethical statements. I don't think I could persuade them to change their position but I couldn't let that go unchallenged. Norms change one person at a time and speaking out is important.
Let's put it this way - the argument above would also apply to saying that a private citizen who catches a thief stealing their catalytic convertor can torture them slowly and painfully for weeks without actually killing them. You owe them no moral consideration, it's a deterrent, etc. Do you need someone to make a persuasive argument against the merits of that to realize that it's abhorrent?
It’s the divvying up of ingroup and out group that tend to create disagreements in morality. I.e. we understand forgiveness, but we disagree who is worthy. We understand justice but we disagree who deserves punishment or why.
That tends to make a pretty big difference in results and conclusions.