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I am very sorry to learn about the care and condition of this poor animal.
Unfortunately, the terrible treatment by the dog's owner does not excuse the fact that you stole the animal, and that he can report you to law enforcement for criminal theft, as well as sue you for "conversion" (the civil equivalent of theft).
You can report the animal's care and condition to animal control, and he can be punished for animal cruelty, as well as having the animals removed from his care (at which point you can potentially adopt the animal through the County Animal Shelter).
But, you cannot simply take another person's property.