Idk I feel like living, growing, and learning from Derek should have given him some idea of the difference between wrong and right.
As far as Felix, I agree, he has ZERO excuse.
Well he isn’t human so we don’t know how mermaids think, plus it’s implied that he grew really fast, Derick was allowing him to form a unhealthy attachment (not saying it’s his fault) then realized too late that uriel was attached. And then along with that prior he was locked up, I wouldn’t treat him as sane even as human or with Derick. But now after all those years I can’t believe he makes the same mistake.
Not justifying killing someone but it was understandable that a mermaid was brought in by a human who helped and natured him all the time, Uriel never really got to meet other people, learn morals or anything, just attachment, so it’s understandable him a mermaid reacted that way AT FIRST however, he’s been surrounded by humans and different innovations there was no excuse for how he treated Felix.