Pretty much every minor and major character in Borderlands shows various degrees of insanity. Most of the population are bandits who are completely crazy. But even the neutral and friendly NPCs show symptoms of various mental conditions, usually of the psychopathic kind. Very few people (like Roland from BL2) seem to be entirely sane.
A good example which shows that Pandora seems to affect newcomers slowly are the audio diaries of Tannis in BL1. She appears to be mentally stable when she comes to Pandora, but the more time she spends on Pandora the more does she lose her sanity.
This made me wonder if the environment of the planet of Pandora does physically affect the mental health of people who live there. Is there any information in the games which supports or debunks this theory?