evil, son of concubine, ghost
孽 centers on something born from a wrong or secondary source: a child of a concubine, an evil or ghostly presence, or a harmful offshoot. The meanings share the idea of a tainted or illegitimate origin.
孽 combines 薛 (a plant name) with 子 (child), historically used for a child born of a concubine, which extended to evil or ghostly connotations. The exact semantic development is uncertain.
The left side 薛 looks like a tangled, weedy plant, and the right side 子 is a child. Picture an unwanted child growing like a weed in a troubled household, bringing evil or ghostly misfortune.
For ゲツ, imagine a ghostly child getting revenge: the sound 'gets' (as in 'gets even') cues ゲツ, and the evil spirit returns to haunt.