Everyone is a sadist or a masochist at least to some degree. We all have the neuronetwork that rewards us whenever we inflict pain on others, or the capacity to derive pleasure out of pain.
Sadly this subjects are taboos, very really discussed because they are being considered destructive attributes of the human nature.
This is largely because there is hardly any healthy example of this two self-preservation strategies.
Often this energy is being realised through a form of catharsis. Through an act of which purpose is to simply create a positive emotion, without any further benefits than the enjoinment you experience in the present.
Some examples of this are: BDSM, fetishes, bullying, enjoying aggressive sports etc.
This is not only a horrible waste, but it is also incredibly unhealthy.
The purpose of sadism is really self defense. Doesn't it feel good whenever the bad guy gets what he deserves, whenever justice is served!?
You can't feel sympathy towards people who are hurting you, because then it would be hard to do whatever you need to do in order to keep yourself safe.
Also masochism is a mechanism through which your body is rewarding you whenever you are in pain, so you don't give up to early before you accomplish your goal.
Both of them are self-preservation strategies.
Whenever society abandons morality, the expression of these two becomes very pathological.
You end up with a lot of abuse, and people who are unable to identify it, and this makes them tolerate it.
So the instincts that should facilitate self-defense becomes misallocated.
Instead of being ruthless against corruption they become bullies. They tolerate the abuse they receive, and then find some one weaker the they are that they can torture just so they get to feel normal.
The same misallocation happens with masochism. Instead of working hard towards a goal, it all being spent in some sort of vice, in a self destructive behaviour.
It is not the fault of human nature that sadism and masochism are being expressed in a destructive way. It is the decadent state that society is in right now, that makes a lot of our primal instincts backfire on us.