Awful. Cant even imagine how the owners must feel, so so sad. Just hope the horses went as quickly as possible Whoever did it deserves the same done to them.
Sound bad, but I always worry about fires and this is another reason why I keep my horses out 24/7.
Similar thing happened a couple of years ago up north. Arsonists set fire to a stables and whilst everyone was trying to resuce the horses, the robbed the house.