I love desire demons - the quote in my sig is
from a desire demon. But what I love about desire demons is their nuance, and how morally grey their temptations often are. Doesn't matter what form they choose to appear in. They could be hovering orange spheres and they'd still be my favorite type of demon.
Imshael scratched that itch for me. If the inquisitor had fought him and died, the world would have been hosed. That's a huge risk to take in order to kill
one demon.
The only consequence is that he kills Ser Michel. Well, of course he kills the person who's been trying to kill him (
unprovoked, I might add) since they first met! A normal human would do that too. In fact, the Turning the Tables questline in the very same region is about a human doing the same.
Minus the red lyrium operation, everything Imshael has done has been some flavor of self-defense. People remember that he killed an entire Dalish clan, but they bound him first. Binding, as in the thing that's so traumatic it can change spirits' very nature.
Even ignoring all that, Imshael is like the Br'er Rabbit of demons. Oh no, Ser Michel, don't destroy those stones! That would
hurt me! Trying to kill him seems like the sort of decision that could have unintended consequences.