TLDR: It's easy to enjoy killing fictional characters when they aren't given any context for you to care about them. But the moment you give them a backstory and a personality, people generally don't enjoy it as much (unless they really hate the character).
(a) What you say is not really true, given the number of shows where the death of characters you care about is part of the selling point (the feelings that said deaths cause are apparently popular, despite being negative). Just in the area of anime, shows like Madoka Magica, Neon Genesis Evangelion, and their following come to mind. (NB: of course they are much, much more than simply violence, but it is nonetheless true that the violence in them, and the fact that it affects characters we come to know and care about, is something that attracts viewers.)
(b) Even if it were true that we enjoy killing only those NPCs we have no actual emotional bond with, that by itself would be more than enough to justify the claim that violence in art is enjoyable. Which is my point. And which I think is a fact that I think people should be interested in, because it does say something about humans.
Note that I'm not trying to say we're all sadistic maniacs who just want to hurt others -- not even in our gaming personas. Nothing could be farther from the truth. I sense that you are arguing to defend the idea of human kindness and ideals higher than "let's turn it all into chaos," and I have no problem with that -- it's also true. We can be (actually are) kind beings who want society to evolve in a good direction. But progress is not achieved by simply declaring that the "bad" (are they really bad?) aspects of humanity aren't important, or definitional, or are just the defects of a few bad apples. It's true that guro porn is a niche genre, with relatively few admirers; but it's also true that the enjoyment of guro porn comes from / is based on other (perhaps equally disturbing) things that are much, much more widespread among humans. To paraphrase Karl Gustav Jung, that which we hide or deny about ourselves eventually comes back to bite us in the ass.
(Let me also say that this idea -- that high ideals that include human kindness, mutual understanding, and a yearning for good -- is very well dealt with in Maggot Baits. Consider its basic metaphor: even though high ideals are all empty and fake, humans who do not have them are like bodies from which all limbs have been severed, i.e. "maggots" [a word with important metaphorical meanings in Maggot Baits], because, even though our limbs are not what define us as humans, their absence is an offense to the human form and what humans can be and achieve. To mention a quote from Maggot Baits that always has me tear-eyed and will probably stay with me forever: "In that living hell where everyone wounded and took from her, there was only one person who responded to her prayers for help. One person who allowed her to keep faith in her ideal vision of humanity and the world she wanted to believe in there at the final moments of her fading life. [...] With everything that is pretty or beautiful in this world... it is always us humans that make them not so. [...] The beautiful, proper way for humanity to be, virtue, bravery, compassion, mutual understanding -- such pipe dreams could only exist within people with the richness of heart to believe in them. [...] That was why humans submerged in darkness could only perceive such kindness as a weapon or poison. It was simply a matter of misunderstanding [...]")