The effect of the kill setting could range anywhere from potentially lethal, where contact with a humanoid target usually caused severe injury or death, to maximum setting, which could disintegrate matter and usually killed humanoids even when exposed to the beam for a short duration.
In 2366, Commander William T. Riker of the USS Enterprise-D was forced to fire his phaser on the kill setting at the Acamarian servant girl Yuta to prevent her from killing the Lornak tribe leader Chorgan. (TNG: "The Vengeance Factor")
When an elderly Kes traveled back in time to 2371, Captain Kathryn Janeway needed to stop her from kidnapping the Kes from the "present" timeline and assisting the Vidiians in destroying USS Voyager. Her telepathic powers had become so powerful, Captain Janeway was forced to set her phaser to kill to stop her. (VOY: "Fury")