I suppose the more powerful a Nevernever being, the more specific their allowed influence. Erlking may kill only during the Hunt, which happens very rarely. Trolls get to do it anytime, but are limited to bridges and naughty children. I suppose if Toot Toot and his kin were strong or pizza-vated enough, they could kill a mortal anytime. Mab's domain lies fully elsewhere, so's Titania's, therefore they only get to do so much when they appear in physical realm and usually need cat's paws to do their dirty work. Of course everything has exceptions, but that's just how Nevernever beings are, nothing's constant and homogenous.
