Battle

So I've come up with this concept that I want to develop further that goes along with my previous regime rant. I keep thinking who would win in a fight, a person who has just been highly trained and regimented their entire life or a natural so to speak. Kind of like a nature vs nurture type of thing. Like if a Green Beret fought a Bear. The green beret has been trained to be a killing machine and the Bear is just built naturally to kill. But I dont exactly mean it in that sense. I can think of plenty of human characters that resemble a regime, specifically Rambo. but I have been having a hard time thinking of a situation where a human character is naturally a killing machine. I would like to kind of develop a plot with this situation in mind but am struggling with the natural killing machine character (which I have been referring to in my brain as the beast). Of course it wouldn't be a straight up 1 on 1 here they go fight type of situation. It would be more of an experiment in nature vs nurture or man vs beast. so what i need is suggestions of a preexisting character that would be a beast or a natural killer or something of the sort. I basically need it to develop that half of the plot and see where this idea could go. ANY suggestions people? a character that could represent the nature of this duo?