3 ideas
Being 'slightly evil' means refusing the social pressure to be reflexively nice while stopping short of actual malice — it is amorality that protects your interests rather than immorality that harms others. The slightly evil actor declines to pay costs that 'nice' people pay out of guilt, but gains nothing from another's loss for its own sake.
Power in any interaction flows to whoever controls more relevant information, so you should systematically reveal less than you know and extract more than you give. Concealing your true preferences, constraints, and deadlines prevents counterparts from pricing their offers against your real walk-away point.
Compulsive niceness is often not virtue but a learned avoidance of confrontation that signals you can be safely exploited. Reframing 'being nice' as a tell of low status reveals that withholding agreeableness can raise your perceived value and force others to negotiate with you as an equal.