"It's alright child, you won't feel any more pain. I'm sorry." SNAP.

The Dreemurs are stronger than Undyne, and we know how insanely strong she is. With that strength and their size, once child is within arms reach like in the pic they can end it very quickly, without any pain or suffering.



OH, also, something important to note, Asgore probably has gone through this before with the Patience Child, and got betrayed. ""Ball" is "Small." You waited, still, for this opportunity, ... then dethroned "Ball" with a sharp attack."