Does anyone know why some enemies turn iridescent when you kill them? Was this some game mechanic that didn't get finished before the game was released?

 

This is usually the result of using the Divine Thorn ability (melee crouched stealth attack triggering the Godlike ability Divine Thorn).

All enemies which don't dissolve into rainbowy petals will end up looking like this.

Ah, OK. I do use that attack as often as possible with my ranger build. It just looks similar to Yatzli's illusion objects, so I'm expecting some kind of interaction is possible when I encounter it.