Doki66's Youtube channel is worth going through. It's not really disturbing more than it is curious.
But for me it has to be a tie. The thing it the wild dog being beaten to death in medico that was posted on this site a few weeks ago
The other was a video that my mate showed me when we were about 15 of a middle aged man being chased by these chavs through a park, he fell over and one of the chavs decided to stand over him with an axe and start chopping at his neck until his head came off. I really didn't want to watch it, I watched through my fingers. I hope it was fake, but from what I could tell it was real.