i would kill my fish by euthanase overdose if i was cirtin they would die anyways. No point in letting them suffer more than they have 2 and cos it's the least painfull way to do.
I always let my fish die a natural death because once in a while a really sick fish will come around after a few weeks from doing water changes/ adding salt/ other.
My next purchase will be some clove oil........but only to keep for extreme cases!! While I hate to "kill" a fish......sometimes its for the benefit of the fish.i.e. they are not suffering anymore. In these circumstances,I believe it to be for the best!!