Shiva

What others do to you is their karma, how you respond is your karma.
You cannot justify sin because of sin. You have a choice to be better, to respond in a more positive way, and to be civil and tact.
You cannot justify your present actions because of past actions; you especially cannot justify your present actions because of past actions from other people way before.
You have to understand that people can change, people can learn, and we're all about second chances. Give them the opportunity to be better themselves, give them the opportunity to change. And what better way to change their heart by being good? By being the example?