The performer. I mean a guy holding a cam isnt' eating shit or sucking a dog's cock, the girl or guy doing it is. Assuming they aren't being forced to do this, they have chosen to get paid to do this on camera, to perform a vile sex act. Keep in mind I say vile without personal judgment, as I find beastiliaty fascinating and uplifting in a personal (not sexual) way, knowing there are ppl out there who do this sick shit makes me realize, yes, even I can get laid.
But without force, the performer is the sickest. The buyer is buying fantasy, but may never indulge in such acts or may just, like myself, be attracted to it just as I am attracted to grotesque murder and injury pics. I don't buy them, I don't use them for sexual purpose, I don't go out and kill ppl after viewing them, so I don't believe a person viewing these materials is guilty of anything, in fact may be repressing sexual fetishes they are unable or unwilling to flesh out. No, this does not apply to me.