When Cooper and Tars figure what Dr Mann is up to in Interstellar, he tries to escape with a shuttle and manually docks the Endurance but he doesn't manage to get a secure lock.
As a highly-trained astronaut he should know this is no good yet he chooses to open the airlock and destroy the ship and, arguably more importantly, to have his innards sucked out.
Why does Dr Mann act so foolishly?
The short answer is that Mann isn't a highly trained astronaut, he's a ground-based theoretical physicist who volunteered to lead this last-ditch mission. His knowledge of space flight is limited to a single mission and his training was done on the cheap by flight controllers who themselves haven't ever been into space.
It's clear that he doesn't recognise the danger. He ignores Coop's verbal warning as well as the master alarm, something a real astronaut would never ever do.
On top of that, he seems to be displaying major symptoms of mental illness; paranoia, psychosis and disassociation, all things that would be likely to lead to impaired judgement.
There is a paradox between Dr. Mann's past (what we hear from Brand and others) and what we see from him in the current time.
When Cooper and others wake Dr. Mann, his first reaction is to cry. He changed a lot and his beliefs changed also. Rather than saving humans and such great goals, he just wants to go back to earth.
He lost his hope totally and years of solitude made him a coward; broken and hopeless. I think Dr. Mann is a character to show how a human could be miserable, villainous, and selfish.
The only thing he thinks about is to come back to earth and he doesn't accept any advice from anyone because in his belief the mission has failed even though others want to continue the mission.
