Is it Asur to watch violent sports like boxing or UFC, or some of the horrific videos of war footage that can be found online (which were pointed out to me, and to which I won't link here) for entertainment?
According to the tana kama in the baraisa on Avodah Zarah 18b, you can't go to stadiums, because of idolatry and because of frivolity. Rabbi Nasan permits it because he might be able to save a Jew if he's offered to the gladiators, or at least he can testify that he died so that his wife could remarry. There's nothing marked in the Ein Mishpat there, so it seems that we don't have someone who tells us whom the halachah follows. It would follow that according to the tana kama, you're never allowed to go to a violent sport game (or really any sport game, since that would still seem to be frivolity, which is one of the reasons); and according to Rabbi Nasan, you're allowed to go if you can save a Jew or testify that he died. (But it seems that Rabbi Nasan's reason would no longer apply since they don't kill people in sports nowadays, and the players probably aren't even Jewish.) Needless to say, CYLOR.