It sounds like she gave the horse the most humane death she could.
So, why is there so much debate about how lethal injection is inhumane because it might cause unknowable pain to the human condemned?
I don't want to get in a debate over whether lethal injection for humans is inhumane, but there is debate in horse circles about lethal injection versus captive-bolt (or simply a bullet). Some States mandate lethal injection despite evidence that the latter is more humane.