Humans are the only animals that routinely display their weapons as a means to reassure. Because teeth are weapons and if a tiger smiled at you, it sure as hell wouldn't be doing so to make you feel at ease.
The animal equivalent of a human smile, jaws either open or closed but all teeth showing and lips pulled back is a snarl.
Chimpanzees have 2 smiles that looks like a human's. If all their teeth are showing, that is a warning, not a smile. If only the lower teeth are showing, then the animal is relaxed.
The construction of a dolphin's jaw gives it an upward curve that makes it look like they are smiling. But they are not.
Dogs and cats smile but with closed mouths.
I suppose a killer smile would be a smile that kills you. Sort of Medusa powers in the mouth.
Iron Noder 2020, 19/30