People that are inherently "good" often take the step towards something evil.  Often times these people know what they are doing is wrong but continue to do it anyways.  Good people often times have certain ideals that they value and would never cross the boundary into what they determine wrong.  In my opinion, good people take the steps towards becoming evil slowly by crossing into grey areas.  Why they choose to take the first step towards evil depends on the situation.

There are many different things involved in somebody choosing an evil route.  Outside influence is the main cause of the initial step toward evil and other people usually play a large part in this influence.  Lady Macbeth has a negative effect on Macbeth by insulting his manhood and pressuring him to make the steps towards darkness.  Lady Macbeth eggs Macbeth on by saying, "Wouldst thou have that / Which thou esteem'st the ornament of life, / And live a coward in thine own esteem, / Letting "I dare not" wait upon "I would," / Like the poor cat i' th' adage?" (Macbeth 1.7).  Macbeth allows himself to be persuaded into doing something bad but knows that he is heading down a bad path.  Good people often choose to make a poor decision because they consider it a one time deal that will not change their character.  They believe that one bad action will not do any harm and that it was necessary for the greater good.  Often times this first evil action won't have serious consequences and the person thinks that they can get away with more and more.  Eventually, the person will have a lot of grey areas in their life where right and wrong blend together and their choices will become harder to determine between the two.  The initially good person will become darker until all of their choices are bad and they don't recognize themselves.

Making evil decisions has an impact on somebody both externally and internally.  I think that internally has a larger effect on the person because it impacts their way of thinking and what they value.  A person's values make up who they are and when they change towards a darker side, the person changes.  Macbeth makes this change into a darker personality throughout the play when he commits darker and darker acts that increase his guilt.  One example of his conscious realizing how he has changed is when he imagines a dagger after he kills King Duncan.  Macbeth says, "Or art thou but / A dagger of the mind, a false creation / Proceeding from the heat-oppressed brain?" (Macbeth 2.1) describing his guilt being manifested as a hallucination of a dagger.  The dagger shows that Macbeth has grown a guilt for killing Duncan and he is aware that what he has done is wrong but has no idea how to stop his downfall.  Externally however the person's life will become drastically different with a lot more darker situations to deal with.