People you dined with, showed them hospitality, showered them with love and care, friendship and compassion, even going so far as to share your innermost secrets and most personal moments with (seeking friendly advice or needing conversation), turning on you, taking advantage of that, treating you like an idiot for being nice to them unconditionally, betraying your trust and spreading sit about you and yours to others, or actively seeking to harm you, destroying other relationships you might have with other people. For me, that is the worst.
Or girls cheating on you, lying to you, deceiving you, even if they were everything for you and you treated them like princesses.
Dishonesty, lies, theatrical acts in everyday life's aspects (however trivial or insignificant they might be or seem to be), cowardice, deception, malice, treachery... Modern society's anti-values and trade coin make me sick to death and angry to berserk. Whatever doesn't kill you makes you stronger, the British say. Then my strength should be well over 9.000 (between serious and joking)...