Who is it? Michael Myers.
Or someone as sick as him.
If this being got to the point where it killed not one animal, but three or four, he's capable of much worse.
If he enjoyed that, of course even if it did not, but I'm sure he did, do the world a favor and take its life and rip it apart.
Even in the case your cats shitted in its food, in no way it has the right to murder them.
I hope they did not suffer, and I hope your dog will heal.
If not, use violence, and don't put any limit in it.
These people are capable of anything, and most of the times they can harm even people, and children.
Animals, in the end, are like children, and it killed your children.
Why don't you ask a Demon to save your dog? I mean, you have all the rights, and your dog has to die when he's old, not for a creep.
And if the dog is suffering, you might use energy to kill its pain.
If its head it's damaged, I would keep an eye on him so that if he has seizures you can stay with him.
My dog had seizures after an infection in his stomach, and my aunt would hold its head and stick his tongue out. I think it might have helped to make him feel not alone.
Do you have the money for a vet?
Is there anyone who can give you some medications for your dog?
I don't know where you live, but if you can you may buy an essential oil without synthetics with a calming effect at least through aromatherapy.
Maybe lavender, but NOT LEMON OIL, not many people can use it correctly on animals.
Frankincense oil can help his immune system, and maybe give him better chances.
If your dog is traumatized, use Cardamom oil as it can boost his mood.
You can also apply these oils on his spine, as it is very common.
But first, verify my advice, and if you want to proceed, it's always better than doing nothing.