Because like you said yourself purpose of punishment is to modify future behavior. Unless we talk about that nonsense of punishment as payment for crime n which case it is like some religious sacrifice.
so punishing yourself makes no sense because it is much more logical to divers that effort to the real issue.
If you are very strongly indoctrinated that if you hurt yourself you will somehow pay for your sins and get rid of guilt then you may be doing it just to get rid of guilt.
Hurting other people makes much more sense because you simply assume that they are guilty for your problems or they are guilty just because they do not care.

If you have desires to hurt others to boost self esteem then yes you probably have some self hate too.
But I just wonder how it is all justified, as what do you think and feel when watching some woman being torture or fantasize about torturing someone

I agree that same source can lead to both desire to be hurt and desire to hurt other, however I think it is not self hatred itself but something what causes self hatred
self hatred cannot exist without reason, there must be something what you feel wrong about yourself and what cannot be fixed.
ant then you can chose what you will hate, yourself or something else.

however if you only have desire to be humiliated yourself it is likely caused by some other reaosn