Nastiest thing I seen was some girl taking a bunch of diahretics, twisting herself so she was lying on her back, bum pointed up into the air... and an arch of... 'crap'... flowing from her bum all over her face...
Beheading videos are gross to but those are more the sound that fucks me up. Like the one of the Russian soldier where they slit his throat and you hear him trying to breathe and it's coming out as more of a rasping noise or gargle. That's blech.
I've not seen one of someone cutting their penis head off but I saw one of a dude putting a rubber band around a finger and proceeding to chop it off then burning the tip at the end. I don't know if it was faked but either way it was sick.
Why would someone do a lot of things? People are messed up. I think someone cut a little finger off on a talk show once because Ripley's Believe It or Not was paying them to do it since it was the longest finger in the world.