What does Dr. Starman finally want to say?

First of all, let's be clear. At that time, Cooper, the hero, would definitely go back to Earth to see his daughter. At present, Dr. Mann has only one purpose in mind, to live. Going back to earth is doomed to a dead end. In fact, Dr. Mann has long known that there is no solution to Plan A, and human beings on earth can only wait for death. PlanB still has a glimmer of hope. He took the main ship to another planet that is said to be suitable for survival, that is, the planet that Anne Hathaway first suggested, and the explorer on that planet was also her lover.

Some people may think that even if they return to earth and wait for their destined future, it will be at least a few decades or a hundred years later, at least this generation is still alive. Can't we just say we'll go back to earth together for the rest of our lives? Obviously not. You want this ship full of the hopes of all mankind to come to Dr. Mann's planet. You thought there was hope for human migration, but everything turned out to be a lie by Dr. Mann. Doctors tricked them into coming to this planet to save him just because they were afraid of death. This planet is simply unfit for human habitation. So even if they finally return to earth safely, do you think Dr. Mann will still be alive once this fact is revealed? Tucson's model is broken.

So Dr Mann is going to kill them all, take the main ship and fly to another planet.