Partly because he wanted to please Jack, and partly because it felt good to finally punish someone like they deserved, the Doctor set about imploding a planet.

He and Jack stood in the door of the TARDIS, high in space above the planet as he handed Jack the trigger. "Give it a push, and this time, nobody lives."

The Doctor shivered, finding it easier to ignore that voice whose scream was finally getting fainter in his mind. This was Time Lord justice, nothing more, nothing less. It's what they deserved for the pain and destruction they caused, especially the pain it had caused Jack. His friend had lost enough people in his life already, and the Doctor's intent was to keep that from happening again.