Kazuya was introduced as the hero in the original game, having entered the tournament to defeat his evil father Heihachi. Heihachi also ran a powerful corporation involved in various nasty deeds, so Kazuya appears totally justified in wanting to beat the tar out of him, which he succeeds in doing.
It turns out in the sequel he made a deal with the devil to help secure his victory, and when he took over the corporation he was even more ruthless than his father. Heihachi returns to get revenge on his son, eventually tossing his body into a volcano. That should be the end of that, but Kazuya’s body is retrieved in the fourth game and then the storyline becomes hopelessly tangled.
It was a bit of a shock for series fans to realise the hero of the original was really a bad egg the whole time, and while he’s jumped from anti-hero to villain a few times he’s ultimately just as evil as his dad.