It would be treated the same as exploiting a system for any other reason. Friendly worms have been done before (welchia). The problems with friendly worms are

It would be treated the same as exploiting a system for any other reason. Friendly worms have been done before (welchia). The problems with friendly worms are numerous. It is more than just a legal issue. A malicious worm is looking to propagate quietly and perhaps leave some sort of backdoor control channel. A friendly worm has to propagate (faster than malicious worms), and patch (without DDoSing patching infrastructure), and self terminate (which harms it's ability to propagate). It's hard to imagine a real world scenario where a friendly worm would be effective. It would either take too long to develop, or it would do just as much damage as a regular worm.