The development of the character 歹 is confusing. Earlier it was written as 歺 , but later as 歹. Subsequently, as 歹, it merged with another character that had evolved to become identical in shape (had become a ‘homograph’).
The original word written with 歹/歺 was in Chinese at one time pronounced è, probably with the meaning ‘corpse’, while the Chinese word 歹 dǎi has the meaning ‘bad, wicked, evil’.
The meaning of 歹/歺 is still visible in characters which also contain that element, such as 死 ‘to die’ and 殘 (originally ‘to harm; cruel’). 歹 in its meaning of ‘bad’ on the other hand, shows up in Chinese word compounds like 歹意 dǎiyì ‘malicious intent’. In Japanese there seem to be no extant word compounds with 歹, except those pointing to its role as 部首 bushu. Those words (歹偏 gatsuhen, ‘bare bone’ signific at the left; 歹部 gatsubu, the characters listed under the bushu 歹) simply point to 歹 as an element in a character and have no real meaning otherwise.
According to Qiu Xigui, 歹 dǎi appeared somewhere around the start of the 13th century. It may have been loaned from Mongolian. If correct, 歹 dǎi is perhaps unique in being loaned both as word and as written symbol. Mongolian was at that time written using the Tibetan alphabet, in which [ta] was written ཏ, which looked a bit like 歹, leading to the homograph mentioned earlier. 
歺 goes back to the period of the oracle bone script with the meaning of ‘bones of a dead person’. Ochiai writes that it simply represents a shoulder blade. Looking at pictures and drawings of shoulder blades, I suppose that’s possible (the hook at the top could be the coracoid process) but even in its oracle bone script shape the graph is already very abstract, and unrecognisable without being told what it is supposed to depict. Below are pictures of oracle bone script versions of 歺:
In some variants the top part 卜 looks a lot like the 卜 on top of 占 (‘divination’; there 卜 depicts the cracks in the divination bones). If Ochiai is correct that must be coincidence, or perhaps stylistic convergence.
Qiu Xigui p. 170-171; see also Qiu Xigui p. 137 and p. 312. Qiu writes ‘sometime between the Song and Yuan dynasties’, but that is a rather vague indication, considering that the Southern Song (1127-1279) and the start of Yuan (1271) overlap (the Northern Song ended in 1127).
Ochiai (2011) p. 72; Ochiai (2016) p. 568.
Tibetan letter [ta].
Small seal script shape of 歺.
Oracle bone character shapes of 歺.
Oracle bone character shape of 占.