A schoolbully (child or teenager) punches, hits or otherwise knowingly injures another child. Person B feels pain. Person A makes no pretence about it. He doesn't hide that he has enjoyed inflicting pain.
A teenager or young adult may commit crimes. He makes no secret that he enjoys hurting another person (whether directly by assault, or indirectly by theft etc.) People have sometimes said that committing crime can the expression of unconscious anger. Unconscious anger is an oxymoron. But where is the unconscious bit?
Sometimes the process gets more complicated. A mother feels she needs to slap her 3 year old child in order to discipline them. This is fairly common in Australia, though most of the slaps are fairly light. (None the less, hitting a child is unlawful in some countries, also I know by experience its possible and easy to raise children without hitting them.)
In some cases though, parents clearly strike their children for selfish reasons. They find it helps them to relieve tension and stress. In other words, they enjoy it because it helps them feel better. When I was a teenager I saw an adult man strike his 2 year old and 3 year old son as hard as he could. Adults who strike their children (hard) often rationalize after the event. "I did it for his own good". In the previous example, the man claimed he hit the child, because he was holding his fork in such a way he might put it in his brother's eye.
One American by the name of Walter Freeman performed thousands of lobotimes on people without painkiller and with a 4% death rate. The lobotomy (This sort of thing was common in the 1950s, Royal Park in Melbourne had 50-100 people dying a week (from psychotic exhaustion!!! which of course does not exist. ).
Walter Freeman not doubt did this for the good of his patients and with the highest of intentions. The point is Walter Freeman is similar to the schoolbully who punches smaller children, but is much more cunning. He enjoys hurting others, but has invented rationalizations so he does not feel guilty about it.
A lot of male and female ritual circumcision/mutilation in some countries is openly sadistic. Gangs who chase down youths who are clearly trying to escape, and forcibly mutilating him, would certainly appear to be enjoying the experience, especially as it had been done to them.
Extremely rarely, some doctors are serial killers. Some of them killed hundreds of people. Unfortunately, because of the tendency of the medical system to protect its own, some of these people were very hard to catch. One American doctor used to poison people. When he poisoned people (nonfatally) he was sacked (rather than charged with attempted murder) and would simply work (and poison) at another hospital.
Let us say hypothetically, that a sadistic psychopath (yes they do exist) enjoyed mutilating children. Imagine the rare person who is a true sadist. What could be better than performing routine circumcisions on countless infants? It is not an accident that people end up in a particular role. Some circumcisors have performed many thousands of circumcisions. Whats more, its legal and they get paid for it.
It is taboo to talk about the possibility of doctors being cruel, but it needs to be talked about. Sadism in this context means enjoyment in any form from inflicting pain onto another, and has nothing to do with sex. There are certainly enough isolated cases of obvious overt cases of sadism in medicine. One simple example is an obstretician who gives an epitotomy (without painkiller) to a woman who expressly refused consent. Another example are premature babies who have been given curare to paralyze them, and had major surgery done on them without any painkillers.
There have historically been many unethical and cruel experiments which include: Leaving men with syphillis untreated so the disease can be studied. Deliberately injecting children with diseased material for the same reason. Injecting a child with radioactive material, to see the effect of it on his body. Many experiments are ethical and are not cruel. These examples are given so show that some cruelty exists within medical research.
It is no secret that some doctors are into power. Sometimes they are selective about what information they give to their patients. The term 'doctors orders' is no accident. In a circumcision, the circumcisor has total power over the victim. Feelings associated to power can sometimes be related to cruelty.