According to the Urban Dictionary:
"I'll rip your head off and sh*t down your neck
A person, or entity, separates
your head from
your body
and thereafter takes a dump that glides
down your neck.
From the Duke Nukem game, said by Duke Nukem (ingame character)." (note: I have edited for swearing and spelling)
So now you know where it comes from. The meaning? It's basically a sort of final insult. Not only do you kill someone, which is bad enough, but you pull their head off and then defecate into the open neck. Not very pleasant, I'm sure you'll agree!
So in the original post, the speaker was saying that what he was planning to do was quite tame compared to what he might have threatened to do.