This question is a matter of opinion but I would say out of pollution, consumerism, discrimination, terrorism the worst is discrimination because it is a way in which humanity divides itself based on irrational things such as the color of someone's skin, the religious and/or ethnical background, gender, or simply because of different opinions.
I would say that discrimination is the worst. My reasoning behind that is because it can break a nation completely. By discriminating religions and races, you can only except madness and chaos to be in effect. People are all the same, so they deserve to be treated equally. I hate to see people being leftout, even in cases of money. That is why discrimination is disgusting in my view.
