I do not believe that what we call self-hate or self-loathing is narcissistic. I do not think it's productive to call it self-absorbed either: if someone punched you in the head, you'd be very self absorbed in the sense that you're focused on the pain that you are feeling. It wouldn't mean you are "being selfish"!

I wonder if self-loathing serves multiple functions? Could it be an expression of deep frustration at our real or apparent shortcomings, or at our tendency to repeat the same mistakes over and over again until we finally learn why we are making them?

Could self-loathing be a defense mechanism of sorts, perhaps to cope with fears of being negatively judged by others? In this case, I can imagine someone essentially saying "I know you are going to tell me I'm a horrible person who should feel guilty about some random thing, so I'll punish myself first - maybe then you won't act as severely toward me". From what I've read in psychology, this could be a bona fide psychological defense, but I'd hardly call that "narcissistic". More like a "superego" that's being warped by social anxiety!

A third possibility is absorbed criticism or invalidation causing self-hate. If enough people asserted that you are stupid or ugly or incomprehensible, eventually you'd begin to wonder if they have a point - even if it wasn't accurate. We are social creatures who pay a lot of attention to what other humans are saying about us.

The reason I think multiple functions are involved is because I suspect this is what MY OWN moments of self-loathing really are.