This is the basic fundamental law of humanity similar to 1,2 ,3 decimal system in mathematics. We haven’t learnt this even after 100000s of years of civilization, surprising!! How come we need to tell this to each other even after so many years of evolution.
There is a clear evidence people that you get back whatever you give other people, yet people harm others. Nobody should make others suffer, if they do they would be subjected similar conditions in future or even worse. We have learnt this from mythological stories from Ramayana, Maharbharat to present day and even from our daily lives.
Looks like in our every day run of the mill life of TV, Shopping in Malls, Movies and making money goals, we seem to loose basics of humanity. Sad 
