This entire post is about this video, and it's pretty short, so I suggest you watch it:
I think this dismissal of diegetic justifications falls (just a very small bit) flat.
Sure, you could write a story where characters are only doing bad things because they think those terrible things are justified.
Or you could write a story where evil is somehow programmed into the antagonists. That could be used to explore interesting themes about regret and self-hatred -- I think even Animorphs got into this with a Taxxon-viewpoint (although I'm deeply uncertain), so it's not too niche.
Or you could write a story where some people take disgusting and perverse glee in despicable acts -- that is more relevant than many people admit to understanding what we ought to do about ISIS, for instance. So all three types of stories could have some interesting thing to say about the world and our place in it.
If someone says "I don't like gleeful and disgusting violence", they are missing out on stories that would inform our ethical intuitions about ISIS. Responding with the in-story explanation seems appropriate -- not to justify gleeful violence, but to give a glimpse into the potential value such a story might have, by making the themes the author was going for more obvious.
That being said, if you don't like something, just say you don't like it.