Answer:
Yes, ordinary people can do evil. Anyone can perform evil acts or act evil. We are animals that have formed a society based around love, but love and hatred come from the same place. Love and hatred (which cause us to do such evil things,) are extremely strong emotions. When we are too overwhelmed by such hatred, we forget our compassion and let rage take over to perform evil acts. When the hatred calms, we see what our emotions have led to, leading to feel compassion for those we hurt. It is also in our natural instincts that we compete with eachother, much like other animals. This is why we are so comfortable hurting eachother.
Hope this helps, sorry if it is too long! :)