ill
痗 centers on the state of being ill or sick. The single given meaning is straightforward, with no broader conceptual cluster to synthesize.
痗 combines the sickness radical 疒 with 每, which likely contributes a phonetic element. The exact historical development is uncertain.
The sickness enclosure 疒 wraps around 每, like an illness that comes every time. Picture a recurring sickness that makes you ill.
For バイ, imagine buying medicine to treat the illness: buy -> バイ, and the sickness enclosure 疒 reminds you of the need to buy a cure.