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 copper and others awake Dr.Mann, the first reaction of him is crying hard. 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 think about is to comeback to earth and he dont accept any advice from anyone because in his belief the misson is failed but others want to continue the mission.