Big grey area, and whilst it's not written in law, the injury they sustained would probably be the deciding factor, as if I turned up and the thief had a bruise and some damage to his pride, I'd be congratulating you and taking him in.
If he had bleeding to the brain from banging his head, it'd probably be you bring brought in, though I would empathise with your position.
It's a tricky one and there is no 'right' answer.