Some UFC fighters, at least from a second hand source, aren't that dangerous. I had a friend who was bouncing in a bar out in Vegas when a couple of them came in together.
They got in a pissing contest with a group of Mexican guys. The UFC dudes were out numbered, and literally told these dumb bastards "you don't know who we are, we will beat all your asses, one at a time."
Evidently the Mexican guys did know. They were able to talk the UFC crew into the idea that they couldn't settle it in the bar and to come out and meet them somewhere around the block (I forget where, it was a park or courtyard or something secluded).
My friend convinced the UFC crew that these guys were not going to have fair, 1v1 fights, but would just knife them as soon as they got out there. The Mexican guys left but the UFC crew stayed in the bar.
Anyway - my point is that the most dangerous bad ass has three qualities -
He possesses an attack that can knock a man out or kill him with one blow.
He can access that attack and use it with so much speed that most people will not even be able to react to it. Some people can unclip and open a knife while swinging it.
He has no inhibitions and is emotionally willing to engage in violence at any time.
Most people, including UFC fighters, can't just go crazy and start trying to kill people without some build up. That hesitation is what will get them killed by a really dangerous person.