Caution: Spoilers (Enhancements?)
Why does Mann try to kill Cooper?
It appears that Mann had to falsify the data in order to save his own life. It does not appear that he should have known that he would be jeopardizing humanity by doing so. He thinks he has to stop Cooper because Cooper is bound for Earth. This is bad because Mann knows that he (Mann) will need all available resources to get the human seeds to Edmonds’s planet. His comments about survival are probably to drive home just how sure he is that Cooper could not be talked out of going back to Earth despite its certain doom.
Why doesn’t Mann just tell everybody that he falsified the data?
There may be insufficient clues in the movie to answer this question. It may simply be that he’s not sure that he can trust them. And, before he can determine whether or not they’re trustworthy, the dominoes start falling. He sent the beacon for anyone to rescue him, assuming that no one would. When the rescue party did come, he reasoned that, given their limited resources, the group could only make one trip (to deliver their payload to Edmonds’s planet) before those resources were depleted.
Cooper [After being pushed off a cliff by Mann]
What are you doing?
I’m sorry. I can’t let you leave with that ship. We’re gonna need it to complete the mission. Once the others realize what this place isn’t, we cannot survive here. I’m sorry.
Don’t judge me, Cooper. You were never tested like I was. Few men have been. You’re feeling it, aren’t you? Your survival instinct. That’s what drove me. It’s what drives all of us. And, it’s what’s going to save us. ‘Cause I’m gonna save all of us. For you, Cooper.
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