Well one thing we would need to know is exactly how they died. But without that information there are a few plausible ways the hands and feet are missing. Just some fun theories! 1. Hands and feet can actually literally 'fall off' of the body first as it decays. This is why often bodies found after major disasters like tsunamis often are missing their hands and feet. 2. Also because hands and feet will separate first with decay, they make easy pickings for scavengers who will take them away to chew on. 3 If the fallen were involved they may have taken the hands right after death or as they found the skeletons. The hands could make good trophies. 4. Fallen may have taken them for their bones to fashion some of that bone jewelry they seem to have on them (I am pretty sure I saw that on some of the concept art). Points 3 and 4 can be seen today in some cultures with a tribal nature (for better lack of a term). I can go into more since I study bones and bone tools but if I were to make a story I like the thought that the fallen took the hands and feet either as trophies or to make decorative jewelry.