Let's look at show-don't-tell first, and why it's a POV issue. Here we are, the happy reader reading a scene, and let's say our character is painting a fence. We're told how hot the day feels, etc., etc., and then we're told our character has a smudge of paint on his cheek. Well, the way this was handled in this particular scene, the character had no idea. The writer just told us. I wasn't in the character's head anymore; I was in the writer's head. POV. Lazy writing.
Now for the violence. Every reader has different tolerances, and if you read enough good fiction you should rub up against your rev limiter once in a while. If nothing is ever too violent for you, perhaps a long vacation in a country far away from me? Okie-dokie.
Violence can cause another kind of POV problem, because rather than being ejected from the character's head into the writer's head (where I'd rather not go in these cases), I'm being flung back into my head. For most violence I can go along with the character, no problem, especially if he or she is reacting in a way I understand. But there is a line where the violence is so egregious or the character's reaction so unusual that I'll pull back out of mental self-defense.
There's the challenge with writing violence. First, it can't be arbitrary. Second, it needs to make sense in the context of the story. And third, I need to be able to get into the head of the POV character, whether that person is the one inflicting the violence, the victim, a witness, or an investigator. Somebody needs to react to the violence in a way I can understand, or I'm sitting back in my living room trying to figure out how I would handle that situation. That is true of all writing, which is why show-don't-tell is so important, but violence has such a high visceral charge that it is more critical. You can easily blast a reader out of your writing.
Just give me a head I can get in and stay in, and I'll stay with you through all manner of extremity.