Sorry, I don't see how anyone (including M. Shadows) could argue that this song is *just* about "the pleasure of killing". Various lines like "scream scream scream the way you would if I ravaged your mind" or "unwanted clemency" or "unmask your desire" make little sense from a pure "pleasure of killing" interpretation.

IMO, it's either about:
-Rape, followed by murder, of a victim. The various lines that indicate pleasure from the victim just be the attacker's belief that she enjoys it. Or perhaps she really does, for some reason.
-A session of some kind of originally-consensual hardcore BDSM sex gone awry when the man loses control and kills the woman.

The general idea of "scream in pleasure (sex) vs. scream in pain (murder)" is definitely there, though.