Here, I mean “sadism” as in the dark tetrad trait, not the sexual fetish. When something like this is a “personality disorder”, that just means it’s a personality trait that persists even when it ruins your life.
There is a logic behind sadism, in that pain causes people to get to the point without so much screwing around. If a person is fundamentally dishonest, untrustworthy, courageous, racist, religious, etc., they are going to display these fundamental personality traits under duress. Sadism is a personality trait which inclines a person to inflict this duress, and thereby discover who a person really is on the inside. These revelations take the form of general sense impressions which resist simplified description, which is why sadists always want to watch and hear people being hurt (the intuitive truth is in the vision itself, not the secondhand account). This is why sadists also enjoy watching someone else apply duress to a third person- they still get all the empirical details, even if they don’t get to decide which direction the experiments go.
The various manifestations of sadism make sense according to this logic: online trolling, social “needling”, the childhood experimentation on bugs and progressively larger animals, and even predicament bondage in BDSM. A sadist is someone who enjoys watching how people handle duress (I suspect they often fetishize “squirming” motions). By contrast, a psychopath who is not also a sadist will watch how people act under duress out of intellectual curiosity, but will not have the same feeling of immediate gratification (except by association to “Aha” moments).
Back when I was hale and hearty, I read something in what was then called the AFOATS training manual that said (and I paraphrase): “Every training exercise must explicitly serve one of the purposes enumerated in such and such published training program. Putting cadets into a situation to ‘see how they react’ does not fulfill any training purpose.” I remember wondering why they felt the need to put that in there. To me, it was like saying “cholesterol is bad for you, so stop putting cheese on your cereal”. Do people do that? Apparently, the sorts of people who do that will proactively seek out lower management and training positions in order to fulfill this desire.
And a second study revealed that, of the participants who rated high on one of the “dark” personality traits, only sadists chose to intensify blasts of white noise directed at an innocent opponent when they realized the opponent wouldn’t fight back. They were also the only ones willing to expend additional time and energy to be able to blast the innocent opponent with the noise.
So it’s not just your imagination.