Okay, so im going to start off by saying, I'm no doctor but even i would think that creating a human centipede would be impossible.
They could have short breathing. Yeah they can breathe through their nose but what if they were crying a lot? Yeah it would cut it off.
Another thing knocking out people's teeth and breaking their knees. It's quite dangerous and they would be in pain and the blood loss would make them slowly die.
Another thing is that if your mouth is sown to someones butt well then it's not going to be a nice experience. And what happens when their digested stuff come through to the other people.
they could choke on it and they would die/
Any way long story short.
You'd have to be mentally unstable to do that.