It is exactly what one wants determines whether he/she is good or bad. In other way, it is motive behind is determinant factor. However person is good and doing good, but if his/her motive behind is not good, eventually he/she can't be good. You yourself. Your inner consciousnesses always try to put you in the correct path. I believe a good person always have a conscious of others feelings, etc whereas someone evil has no conscious of their behavior whatsoever no matter who they hurt or what they've done.
