I think your best argument is that animals kill/mutilate each other all the time so that a violent death for fun isn't really something that they wouldn't expect or be outside the realm of what they themselves would do.

Okay. But if I were to tell you "Well humans murder each other, so I can murder humans!" Does that make sense to you? Why don't you hunt skin and gut humans? There must be some reason, right? Because you'll be punished if you do? Because we feel pain? Because we're intelligent? Because we're kind? All those are also true of at least some animals you mentioned above.

Life seems a strange thing to to me. At some points, it seems like the one thing that separates humans from animals is our willingness to work together nonviolently, cooperatively rather than competitively with each other, and to regress and just "kill for fun" doesn't feel right.

Civilization has advanced in fact to the point where killing animals isn't necessary in order to eat.

Moreover there's many ways to build one's ego that are constructive rather than destructive. A lot of people in this thread talk about great "hunting family memories." I have awesome family hiking and bird watching memories that didn't require killing in order to have fun.

Do I have fun killing in violent movies, video games, and board games? Sure. Because it fulfils the instincts in a non harmful way.