You treat someone bad once, and to escape having to admit you’re the type who treats others badly you alter your perception of that person until you’re sure she had it coming. Psychologists call it cognitive dissonance, this experience of realizing you’ve done something you can’t admit you’re capable of. Memories and perceptions are shockingly malleable, and the dissonance forces us to adjust our view of the other person so what would have been bad treatment seems completely justified. But then your after-the-fact rationalization becomes a ready excuse to treat that person bad again. Add to that excuse any further justification provided by his attempts to fend off your bad treatment and you get a snowball effect. The more you abuse this person the more you hate him. The more you hate him the more you’re given to abuse him. I’ve heard an expression that the Germans couldn’t forgive the Jews after World War II for what the Germans had done to them. It’s like that. The profoundest hatred and the most inhuman atrocity can begin with tiny acts of uncharitability. I suppose the reverse is probably true also.