Adrenaline junkies are a real thing. Though the actual term is a bit of a misnomer, as far as I know, there’s no actual chemical or psychological addiction to adrenaline itself. Adrenaline junkies are a variety of sensation seekers.
Quick caveat, it’s been a long time since I took a psych class, so I’m a little rusty, and piecing things back together from memory and the internet, mostly the internet.
Sensation seekers are individuals who are unusually drawn towards constant, intense, sensory stimulation. This can lead to a pathological attraction towards dangerous and (subjectively) exciting experiences. Which in turn leads to the individual engaging in risky behavior. That may be gambling, extreme sports, violence, or any number of other behaviors.
It’s probably worth adding, sensation seeking is an entirely normal component of human psychology. We’re all driven to experience new things to one degree or another, and the sensation seeking impulse appears to be affected by hereditary factors. Put more simply, your brain is wired to poke you in the ass occasionally, and get you to try something new. What differentiates most people from a sensation seeker is how pronounced that impulse is.
In severe cases, sensation seeking behaviors absolutely affect daily life. The impulse to push boundaries or engage in “unsafe” behavior is more pronounced, meaning they’re more likely to blow through stop signs or speed while driving. It can (and probably will) affect the kind of media they consume. More aggressive content will, usually, win out over other alternatives. Be that TV, music, movies, whatever.
This is reinforced by a euphoric high that the individual experiences when engaging in risky behaviors. It’s not exactly like drug addiction, because it’s their own brain chemistry on a rampage, but the resulting behavior is similar.
The problem with engaging in risky behavior for extended periods of time is, eventually, your luck will run out. Rather obviously, the consequences will depend on what stupid thing the sensation seeker was doing this time. As it turns out, illegal street racers tend to have different outcomes from bungee jumpers. Though, being turned into human tomato paste is a legitimate option for both.
The problem with frequent hand to hand combat is the significant strain it puts on the body. I know we’ve gone over this, but fighting is painful. Even if you win, you’ll still walk away with injuries. Injuries which need to heal before you go back in for more. For someone with a pathological sensation seeking drive, they’re not going to wait, and (given time, and enough violence) the consequences will accumulate.
This is ignoring the legal issues involved. Someone who is pathologically drawn to fighting will run afoul of the law, quickly. This may simply be from the fights themselves, or the individual may escalate to increase the high experienced.
Before someone suggests a character who restricts themselves to professional or semi-professional bouts, remember a major element of this is the desire to engage in risky behavior. This includes not taking enough time between bouts to recover, because the impulse is to get back in.
Remember, this is a fundamental personality trait. While the behavioral loop, and some elements of this are superficially similar to an addiction, this isn’t one. Someone does not go from normal to being a combat adrenaline junkie. They may go from extreme sports to drunken brawls, but the underlying elements that lead to this kind of behavior are already there. That doesn’t mean people in their life were aware of the risk, however. Self-delusion can be a powerful drug.
While it can be a little difficult to fully unpack, I’d recommend taking a look at Fight Club by Chuck
Palahniuk. It’s not an easy read, and is a book that’s remarkably easy to misread. Palahniuk has a very vulgar approach to violence, which suits the subject matter, but might not be something you feel like parsing.