To hurt someone is easy. To overcome the mental obstacles/morality/fear whatever to do that is the hard part.
Some people are just ruthless. They just don't care. If they hurt someone more than they should, well, so what? There are people that are raised this way and feed off it. It makes them better fighters in a sense.
Everybody is born with a natural fighting system like animals are. The kicks, headbutts, biting, punches, etc. you describe this guy doing is a combination of that natural human fighting system and exepeirence/lessons he has learned "on the job" in streetfights. It is a good basic fighting sytem and is meant to defend oneself. But, most of us are not "animals." We think, feel, care, the ones who don't as much tend to sharpen it's animal brutality.
Most of us grow up to care somehow. We get in trouble if we hurt someone. We start to care because we don't want to get punished. There are some people who have some type of moral system or religion that forbids them to brutalize people so easily that stops some. Some people are naturally "lovers not fighters" and don't pursue hurting others too. So, we develop mental restrictions from go "psycho" on someone and developing our inner beast.
Martial arts are more to gain skill. The people with moral restrictions need this skills to overcome animal brutes like streetfighters. It's also to make natural fighters more deadly too (but, that's largely a misuse of martial arts except for military situations). Martial arts is almost alwasy largely for self-defense or sport hardly ever for aggressive attack.
But, some people grow up rough, in usually an uncaring/cold/heartless/brutal atmosphere and feed/grow off it. They become very dangerous but, do turn into monstrous animals usually. That's why a lot of guys like this, just like the animals they become, finally find themselves locked up in a cage with a leash (prison) to protect the rest of civilized society from them.