@Hell_Satan,
First, neither of the people saying he should've killed the vampire said he should've did it without batting an eye. You came in and said the following:
So your fine with MC killing little kids, I'm fine if he kill women and men if necessary but I draw the line with kids even if their 100 years old.
Then when Kayriel replied that she's a vampire that looks like a child you switched to:
Still having him kill someone that looks like a child without batting an eye is like him having no problem killing actually children. Similar to the people who are attracted to woman who look like a child they really have some tendency on that direction I just leave it as long as their in the right age but I draw the line in killing.
I replied that if the MC or character in question is aware she's a vampire and not a child then it's no longer a problem unless the writer wants to write in some internal turmoil for the MC. I pointed out that in wars and conflicts on our planet people, yes they're soldiers, but the point is they have to get over their hangups about killing children if they want to survive that situation. It's a valid point whether he's a soldier or not. If she was an assassin capable of putting into action her mission and threaten his life then he would've had to make that decision.
Second, you're generalizing people. People react in different ways and people handle trauma in different ways. Some people can experience a traumatic event, go through it, and come out of it unscathed while others can come out of it unscathed and months or years later the traumatic effects manifest. This is reality as not every soldier in combat experiences PTSD. All in all, people are wired differently and it doesn't necessarily mean something's wrong with them or their immoral or anything. It depends on the context.
So in one moment, you were arguing MC should never kill a child and that's where you drew the line. You were fine if they kill men and women but no children. When you were challenged that the character in question was a 300-year-old vampire, you switched to MC should not kill characters that look like children without batting an eye. I guess you've never read or watched anime in which there are child-like murderous characters that take life without batting an eye. I can list off a few for you:
Rory Mercury from Gate
Tanya Degurechaff from the Saga of Tanya the Evil
Hansel and Gretel from Black Lagoon
Reisuke Houjou from Mirai Nikki
These are just a few characters with child-like appearances but don't have a problem murdering people. Yes, I'm aware three of these stories are nowhere near the tone of this one, but the point is that you want to put limitations on MC's with an older appearance but cast a blind eye over characters with a young appearance who kill both young-looking characters and old-looking characters without batting an eye.