Honestly i see zero reason why killing a monster with magic is different then killing a human monster with magic

It's about the nature of mortal magic. It's supposed to come  from mortal life itself, so using magic to kill a mortal goes against its purpose and nature, it perverts it, forces the natural energy to do something that's completely against what it represents.

Magic is not composed by the energy of non-mortals, so using magic against them doesn't corrupt it, and it's ok from a... theological? point of view.

I'm guessing using seelie magic against summer fae might also be considered as a sin by the summer court, for the same reasons.