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 , first reaction of him is crying hard.He changed lot and his beliefs changed also.Rather than saving humans and such great goals, he just want to back to earth.
 he lost his hope totally and years of solitude made him coward,broken and hopeless. I think Dr.Mann is a character to show how a human could be miserable,villain and selfish.