I imagine BASE jumping is close to number one if you count it as a sport and define violence as face-planting the ground at terminal velocity. In terms of death vs participation I should think Formula One is right up there even though fatalities are very rare in current times.
There are several sports that could be considered violent, but one that often comes to mind is mixed martial arts (MMA). In MMA, fighters use a combination of striking and grappling techniques to defeat their opponent, often resulting in brutal knockouts or submissions. The physicality and intensity of MMA matches can be quite extreme, leading many to view it as one of the most violent sports.
Some other sports like boxing or rugby also have high levels of physical contact and aggression, making them contenders for the title of most violent sport.
For me, I find American football really violent. An opponent can just clip your legs just for you to fall and for him to get the ball, and to think you would be running at very high speeds.
It is scary watching this video. I don't think I am going to get involved in this type of situation because this is a moment that can get someone killed.