When you attempt to take over control of a botnet, you are committing an act which is just as criminal as what the original owner of the botnet was doing. No matter how good your intentions are, you are taking control of other peoples IT systems, which is illegal in most parts of the world.
Report the botnet to the authorities. When you don't believe that they have the necessary know-how to deal with it adequately, you could offer your services as an IT security specialist to assist them, but do not attempt to do anything not coordinated with law enforcement. As an IT professional you are likely not familiar with proper police procedure. When you just run wild, you will likely destroy or invalidate evidence which is necessary for finding and prosecuting the people behind the botnet. That will not just prevent them for receiving their lawful punishment, it could bring you into legal trouble as well for inadvertently helping them to escape conviction.