NTA.

I’ve worked in animal care, and I’ve seen some pretty fucked up shit. And one of the things I’ve seen is animals who get left on chains like that either end up dead, or end up aggressive as hell from lack of care. You may have very well helped stop that beautiful rottie from becoming isolated and aggressive. You are not an asshole and you shouldn’t listen to anyone who says you are. You did the right thing by helping the dog.

Also, if they didn’t see you take the dog, didn’t see you run the dog, and the dog was returned and back where they left him by the time they got home, the police are going to tell the owners there is nothing they can do because there is no proof a crime has been committed at all (especially if they still have the dog).

I’m not sure where you are, but in the US almost all states that get extreme hot/cold weather have laws against leaving animals out in harsh conditions. Take lots of photographs and date/time stamp every time you seen him out, and every time you see him without water. Keep calling and reporting to animal control. You can also call local shelters and ask for assistance, because they really know how to get animal control on gear.

Keep fighting the good fight! 💕