This is quite a confusing video. By far the biggest question arising from it is why is the cat kneading the other? That's something that kittens do to their mother's stomach when they suckle, and older cats sometimes keep doing it when they're really happy. Improperly weaned adult cats may suckle and knead their owner or even inanimate objects as if they were their mother. According to wikipedia, cats may also sometimes hump an object they are kneading, and there seems to be a very short bit of that on the video too (0:50). Cats can also knead the ground or whatever they intend to sleep on, and the kneader does end up laying next to the other cat.
I think what
>>984421interprets as biting might actually be suckling. The cat is not biting on the scruff of the neck but rather the side, and there is no humping behavior while it's doing this, rather more kneading. It wouldn't surprise me if the dead cat was the mother of the kneading cat. Whatever the case, it seems like the kneader is going through a regression into kittenhood for whatever reason.