There's nothing wrong with having BDSM fantasies, rape fantasies, murder fantasies, torture fantasies, or any fantasies, for that matter. Fantasies are fantasies; they aren't real. Extreme domination/BDSM films are 100% okay for him to watch, because they involve consenting parties. Consensual toilet porn is fine. Trust me, I know people who are interested in some weird stuff, and it's perfectly okay.

There's nothing wrong with watching gay porn, either, and watching it doesn't make him gay. I know gay men who watch lesbian porn, lesbians who watch gay porn, straight women who watch lesbian porn... you name it.

Basically, fantasies are okay and the films are okay if all parties have consented.

However...

Rape, snuff, hidden cams, toilet porn (if it's not consensual), and animal murder films are VERY problematic. It's okay to have fetish/kinks, engage in BDSM, and enjoy watching pornography of that variety. All of those can be enjoyable, and plenty of people dabble in them. Some of the things your boyfriend is watching are not BDSM at all. As Azafadaza said, there's something very wrong with this, because this doesn't involve consent. The murder victims did not consent to being killed. The rape victims did not consent to being filmed. Saying that it's okay for him to watch rape and snuff films is like saying it's okay for him to watch child pornography.

I honestly don't know what you should do, but people who say this isn't a cause for concern are entirely wrong.