You d still be in legal trouble. It s best to report the abuse to the police or animal control asap--they can take care of abuse allegations and do it within the law. Somebody who abuses an animal doesn t deserve an animal but there are still laws that are in place. Vote and lobby for tougher animal protection laws as some states are way behind on those things. In fact, in my state there used to be such weak laws that the police would hesitate at taking a dog from an owner due to fear of being sued by the owner, despite plenty of documentation of health condition, witnesses, etc.
At the animal shelter I worked at, somebody brought in a terribly neglected dog and said it was laying in the middle of the road. The owner showed up half an hour later and took it away. It was in terrible condition, half-bald on its back with embedded fleas, collar crusted thick with mud and rust (had to cut it off), skinny (all ribs), and very ill. Though our shelter had a lawyer on its board, we had no legal rights to retain the animal. We already knew the cops wouldn't help at that time. So even if a shelter can't help save the animal, you certainly couldn't. The rescuers were neighbors who were trying to make a covert move by bringing the dog to us, and I honor that choice, but it would have maybe been better to have the cops come to the house first and make a report. Taking photos of the neglect and evidence of dog's housing, physical condition etc, could help the dog as well. You will not help the dog by stealing it or transferring it elsewhere to somebody who might be able to help via your own efforts.
In cases of extreme neglect and abuse, we were able to sometimes get the owner to surrender the animal, and in bigger cities, etc, ASPCA can sometimes get that done better than a smaller shelter can. Cops and shelter people show up and try to get the person to surrender the animal under some pressure. If the owner is mentally incompetent or ill, a family member once signed off so that 2 dogs could be surrendered to us. The dogs were chained at a tree for a month at a time, with a bucket of water and a slashed-open big bag of dog food between them. They wanted to tear each others throats out, peed themselves if they saw a woman or man with very short brown or dark hair, and had to be carefully re-homed. One had long hair and had to be entirely shaved down. Both were very good loving animals but you need somebody to help you, somebody with the power of the law and authority to try to convince the owner if needed.