basic principles of drama - one of which is "If characters did the most correct thing in every situation, there would be no show".

That doesn't really have anything to do with OP's point though. OP isn't objecting to the fact that the character's actions aren't perfect, they just don't find the character's actions plausible. Imperfect actions can be plausible and perfect actions can be implausible.

In the heat of the moment, it makes perfect sense for them to not immediately kill the object they had previously seen as their own daughter - for reasons I shouldn't have needed to spell out.

OP literally said that this wasn't the part they had a problem with, so I don't know why you're acting as though this is some sort of rebuttal.

Your supposed "weak points" are a juicy combination of you not thinking things through before commenting and not being aware of basic principles of drama ...
... for reasons I shouldn't have needed to spell out ...
... So there ...
... a reminder that you could have intuited all of this yourself if only you understood the purpose and form of drama.

FWIW I don't particularly care about either side of this argument, I just wanted to say that this argument didn't even need to be an argument. It could've been a normal conversation if you hadn't started and continued the conversation with a dickish and condescending tone, and moreover being dickish and condescending while not even understanding the comment you're replying to.