"Kindness. It doesn't cost a damn thing. Sprinkle that shit everywhere!"
Truer words could not be spoken about the act of kindness. Choosing to be kind to someone doesn't cost anything in comparison to the opposite: choosing to be mean. You can choose to be mean, but in doing so you may ruin a friendship/relationship/your own reputation. So in the end you've paid a very high price, and is it truly even worth it in the long run? In my head the answer is an easy no.
People are capable of being nasty and are able to treat others with little to no respect. Don't let how others treat you dictate who you are. If someone doesn't respect you, you should still respect them. Never mirror how someone treats you if it's a negative to begin with. That does absolutely nothing but make you the same as them. Do you really want to become a negative person just to get even with another individual? Or would you rather be a positive energy whom people enjoy to be around as you do nothing but uplift those close to you?
You need to stay true to the real you. Ask yourself the question, "is it worth becoming that kind of person just to get 'even' with someone like that," or would you rather edge yourself away from such negativity? Negative people are a burden to be around. They do absolutely nothing but bring you down to their own level. Either speaking lowly of others, or doing nothing but talk down upon everything in their life. Life can be hard at times, but you are still in total control of how you view it. There may be more negatives at the present, but there will always be at least one positive- no matter how small. I would much rather focus on that one small positive which will uplift your spirit, than the negatives which will do nothing but depress you and cause you (and possibly those around you) emotional harm.
I'm no shrink (or mathematician) but I can tell you one thing for certain- life is 1000% better when you stay true to yourself, and choose to be kind over being cruel.