foul and poisonous in confusion, out of harmony
沴 centers on a state of foul, poisonous confusion and disharmony, like stagnant water that has become toxic and out of balance.
沴 combines 氵 (water) with 㐱, which likely contributes a phonetic element. The exact historical development is uncertain.
Water 氵 mixed with a tangled, messy shape 㐱 suggests a foul, poisonous brew that is out of harmony.
For レイ, imagine a ray of light trying to pierce the foul, poisonous water but failing: ray -> レイ, and the water remains murky and out of harmony.