The ray can be used as intended without killing people, so can bullets. Unless in the US you only sell bullets to people expressing a desire to use them for defence with lethal force, it is not even true to say that the purpose of a bullet is to kill. The purpose of a bullet is to be fired from a gun, at what that gun is pointing is immaterial to the bullet.
I see no difference in a non-lethal weapon that can be turned up to “kill” and a lethal weapon that can be turned down to “stun”, except perhaps efficacy.
