The Joker in The Dark Knight is the extreme example of what a psychopath is: someone who essentially feels no empathy or remorse. That's my guidebook for the psychopath playthrough. You play as a person that doesn't abide by any rules whatsoever. Someone who's only concern is the fulfillment of their own goals with emphasis on the ends justifying the means.
I believe the trope for that is Chaotic Evil.
That's how I play evil. My evil characters don't just arbitrarily massacre everyone, they kill who it's in their self interest to kill and don't care about it. If it suits their purpose to help someone they do, but they'd just as easily kill them if that served their interest more.
I wish they'd sort out the karma system though. My evil characters end up with high karma every time. E.g. kill that Holmes fella to steal his star caps, which s pretty evil - no karma loss. Kill a fiend who's trying to kill you anyway so you really have no choice - karma boost.
I think you should have to work to get good karma - like turning down rewards, being consciously selfless.