I don't understand what's with the villain twist.

It's pretty much clear as day the kidnapper isn't going to be the executives from the first scene of the film. They go there with the intent to get Gondo on their side and seem bewildered and furious when he doesn't comply, so it wouldn't make any sense that they would have an elaborate kidnapping scheme for 30 million yen in the works. They don't know that Gondo has an ace in his sleeve either, and think they can just easily oust him from the company anyway, so some crazy plan like that wouldn't be needed. It would also be very silly of them to immediately kidnap the kid after leaving the house, don't you think? Not to mention they should probably actually know what Gondo's kid looks like and would have gotten the right one.

The movie isn't about corrupt businessmen, that whole aspect of the film is far from the focus. It's not supposed to be a "twist", or anything like that at all.

The movie drops not so subtle hints that it's likely the work of a sociopath acting without reason. Nakadai's character blatantly says that the ransom amount is preposterous and unheard of, and that the criminal is probably mentally unstable. The criminal on the phone also seems to be on the sadistic side, spying on the house and acting very smug. A random hired thug probably wouldn't be acting like that, and the company executives probably don't have time to sit around and terrorize Gondo from the slums over an amount of money that is soon to be trivial to them after they take over the company.

As for it being "completely thrown it at random, last minute" - that is simply absurd. The name of the film itself is a reference to the relationship that Gondo and Takeuchi have. Takeuchi being Low, or in "hell", with his resentment towards Gondo being High, or in "heaven". Takeuchi's resentment for Gondo and his motivation for the crimes he commits is born from this as he believes he is suffering down below while Gondo enjoys a life of luxury up high. He even says something at the end before his execution about how he doesn't want to end up in heaven out of spite for Gondo.

I was misled twice before the real kidnapper was identified as acting (almost) alone. First I thought that Aoki might have kidnapped his own son because. Nothing really pointed to this but I just had a hunch. Second, I thought Gondo was involved because he lit the real villain's cigarette outside the shoe shop while the police were parked illegally across the street.

I think you guys watched way too many Hollywood movies with melodramatic twists and endings. It's perfectly fine and 100x more realistic that the culprit is some random guy. The corporate war at the beginning is just a context to make the main guy struggle with paying the ransom.

There's nothing random about it at all. The class bitterness in Japanese society is the focus of the movie, it's what the movie is about. The kidnapping and manhunt are just the vehicle for it. Gondo's business associates being responsible for the kidnapping would be an entirely different story.

This was basically a police procedural, which was a common type of movie in that period. This one had a little extra twist with the business machinations, but what one is supposed to be enjoying is just how hard and smart the police work to bring the bad guys to justice.