Grotesque

A young couple Aki and Kazuo are snatched off the street while having their first date as a romantic couple after a few years working in the same office. They wake up shackled in a basement that has plastic-covered walls. With no further explanation, a sadistic madman degrades, tortures and mutilates them. He punctures Kazuo's belly with a screwdriver and slices his tongue, then sexually assaults each in turn, forcing the other to watch. Sometimes, he stops the torture to provide medical assistance and cure the couple's wounds, so they can continue alive for a long period of time. This way, he cuts off all their fingers, makes collars with them, pops out Kazuo's right eye, removes the girl's nipples and cuts off her right arm.
As the torture progresses, it is revealed he is simply doing it for sexual stimulation, and tells the couple he wants the two to survive. He finally castrates Kazuo, claiming he has found all the sexual relief he needs, so no longer needs the couple's "services". The couple is moved to a room which resembles a modern and clean hospital room, where the kidnapper takes care of the couple's wounds. It gradually becomes apparent that the man has professional medical training, refined manners, taste for classical music and good wines and dresses in expensive clothes when he is not in surgeon dress. He even mentions he is a wealthy man, which suggests he may be a reputable surgeon, not merely a violent sadist, looking for an extreme way to obtain satisfaction in his lonely life. The couple also notes the doctor has a particular rotting smell always present behind his clean and elegant appearance.
After several days healing, the "doctor" simply tells the couple they will be free to go, he will turn himself to authorities and, as apology for all the suffering he inflicted to the couple, he will give them all his fortune which seems to be very large, as compensation. After all the horror, in a moment alone in the hospital room, Aki and Kazuo promise to support each other once they leave and become a couple. It is unclear if the madman is playing with the couple's hopes to survive; immediately after telling them they will be released, the couple is taken back to the basement. After being drugged, they are shackled again, exactly as they were the first time.
The "doctor" announces they must participate in one final test of love. He pulls out some of Kazuo's intestines and attaches them to a hook. If Kazuo is able to cross the room to the other side (pulling his entire intestine out of his body in the process), take scissors and cut Aki's ropes to release her, both will be freed. However, Kazuo fails due to blood loss (it is also revealed that the ropes restraining Aki have a metal wire running through them, rendering them impossible to cut with scissors; the task was therefore impossible).
Aki begins to insult the doctor and says that despite his classy appearance, he has a skunk odor. Angered, the doctor cuts off Aki's head. The head lands on the doctor's neck; she bites him with her final breath. Kazuo, not dead yet, stabs him in the foot with the scissors as a supreme last action. The couple then dies facing each other. In the epilogue, the madman is revealed to have survived, although he cannot walk properly. He is in a quiet forest where he respectfully buries the couple next to each other in a traditional Japanese way, leaving the scissors on their tombs as a symbol. The next scene shows him in the same car he used to kidnap the couple, covering himself with lots of perfume to hide his skunk stench. A girl walks by, and the screen cuts to black as he goes after his next victim.

Question: As the torture progresses, it is revealed he is simply doing it for what kind of purpose?
Answer: Sexual stimulation