Some people believe that the evil eye is the curse of misfortunes brought about by negative spiritual energy. In that case it is believed that a person channels a lot of negative cosmic energy upon someone who has caused them hurt or pain. The eyes are believed to be the window to the soul and therefore the negative energy is transmitted through the evil eye.
First you gotta make someone mad. Like, real mad. Like, you killed my dog mad. Then you call 'em some bad names, and there you go! Now all you have to do is walk by that person and they give you the good 'ol evil eye.