As for spanking... this is an area in which I have some expertise.
Though it's not a fetish I have, personally, I'd like to add.
Spanking's often nothing to do with S&M, or necessarily even the dom/sub thing. It can just be about a pleasurable feeling caused by "heat" of the spanking in the nethers. Sometimes it's about reliving a pleasurable feeling experienced while being spanked as a child. I knew a couple of really old guys who never quite got over the thrill of being spanked by their nanny.
Of course, it can be a part of S&M and dom/sub play. But not always.
Also, a person can be a sexual sadist without feeling driven to kill a person, or seek unwilling victims. Some just can't fully feel satisfied without inflicting pain on someone. Lucky for (most of) them, there's people who are aroused by having pain inflicted by somebody else. But that's not to say that if there wasn't willing 'victims', a majority of sadists (even some who go to extremes) would go hunting for unwilling ones.
The sexual sadists who do go hunting for unwilling victims are most often also sociopaths with gods know what other comorbid issues. Being a sadist doesn't necessarily mean you're a sociopath. But a sadistic fetish/compulsion combined with sociopathy is really bad news.