How to deal with unkind people

This is a letter I wrote to my father, who in addition to being a Pastor, is a Marriage and Family Therapist.

Dear Dad,

I have a life question. So there is this girl. You have heard about her before. She was the same girl that listed all of her activities at the end of her emails (remember i was stressing out thinking i was uninvolved because of this email). Well I saw her today (she is working on campus as well) and she was carrying bags up the stairs and they looked bulky so I offered to help. So to be friendly I initiated a conversation. "So are you working here this summer" Her: "Yes im working in the CASA office" (normal human would have said something like...what are you doing this summer?) instead we walked in silence and I was barely thanked when I helped her all the way to the car outside. Person inside the car was very kind and she thanked me. However my first initial reaction was wanting to text sam (who was also here during the "im not involved enough" crisis and already does not particularly care for said girl because they wrote together at the newspaper and he reported to me that she is not the best person to work with) so i wanted to text sam "oh my gosh remember (said girl)? Ya i just ran into her and she is SUCH a B." ("B" here stands for bitch and somehow i think that if i just use the first letter it is somehow less bad that im calling someone that... i dont even agree with my own logic now that i write it out) but what is it about people like this that they are so unkind that they promote unkindness? I did not text same because i was lucky enough not to be too hot headed in the moment and sit down and write to you instead because now im more concerned at how quickly someone elses unkindness sparked unkindness in myself. i dont want to be that unkind person. i want to kill them with my kindness even if there is no reciprocation what so ever. i dont like that people i barely know have control over my emotions like this. HELP!! WHAT DO I DO!?!?!?

thanks MFT Dad :)
Love love,
Me

Any ideas??