I consider a pure sadist, in the BDSM context, to be someone who gets off in some way from hurting others, regardless of whether or not that person likes it, and even prefers it when they don’t – or at least doesn’t let the bottom’s not liking it or not being able to take what gets dished out get in the sadist’s way of doing it anyways.
As long as the bottom agrees to those parameters, everything is copacetic.
There’s nothing wrong with consensual nonconsent – and many people definitely get off on it. But it’s *got* to be agreed upon in advance.
That consent is also the only thing that separates BDSM-type sadism from the form described in the DSM that is considered pathological.
If there is no agreement that pushing those limits to the point that the bottom really cannot take it is OK, and especially if it’s stated in advance that it’s *not* (and is therefore a hard limit), then it becomes abuse instead – or even outright assault and battery.
It can certainly take some time to learn where those limits actually are, but once established, if not respected, then deliberately exceeding them (except by obvious accident) becomes pretty much a nonconsensual act, even if the overall play is consensual.
Obviously this can also take some real technical skill to avoid stepping over those boundaries in some cases – as well as a desire and intent to respect them. It also takes open communications with one’s partner to determine when those limits may have also changed for some reason, especially things like the time in the menstrual cycle, any health issues, etc.
A pure sadist of the BDSM type may or may not also be a dominant as well, but often is not.