The spider thing is interesting though. It would seem that being eaten by her children is an adaptation, not just an unfortunate consequence of harsh reality. The strategy appears to be: give my nutrients to my offspring, for they carry a part of me, and the survival of that part is more important than the rest.
Given that most of the cells in a human body are passengers, and not cells with human DNA in their nucleus (if they have nuclei at all), it seems pretty likely that something, somewhere, is making the same kind of sacrifice, and using a hungry human as the vehicle. And I think that ending a life on those terms falls in a different domain--both from an ethics perspective, and from a "how adversarial of an environment do you want to create for yourself"-perspective, than mere predation.
Strategies of that sort are more popular outside of the animal kingdom than in it (I think), which I think justifies a "different rules for different things" approach.
There are parasites for which being eaten is a part of their life cycle.
https://en.wikipedia.org/wiki/Leucochloridium_paradoxum
Black widow spiders are so named because the females eat their mates. Some spider mothers become food for their own children.
Not to mention that (while not exactly what you asked for, but I think it's important to recognize) carnivores kill other species as a means of survival, and even their own species at times. Wild wolves cannot feed their pups without killing other animals. Death is common in nature.