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?
From a responsum of R. Yitzchok Abadi (Ohr Yitzchok vol. 1 YD §26) it would seem to be prohibited on the basis of an opinion in the Talmud with regards to images forbidden to gaze upon (Meg. 28a). Additionally, according to his presentation of Rambam (AZ 10:4) it would certainly be prohibited to watch combat sports which is behavior that goes against Jewish ethos.