Witch-slapped: Macbeth's fall from grace

Why do people who are not "evil" take the first step into evil? What, for instance, is involved in taking that first step "down the primrose path to the everlasting bonfire" (Macbeth, 2.3)? What are the consequences of the individual choosing evil (particularly the internal consequences)? Use examples from the text to support your opinion.

The most fitting thing to start off this blog post would in fact be a quote from The Dark Knight:

And during the subsequent fifteen minutes while I searched for other pictures, I came across this Urban Dictionary post (don't worry Ms. G, this one's school appropriate). It outlines three categories of possible first steps, which are:

Falling to the same corruption you sought to eliminate. (Anakin Skywalker from Star Wars, Lois Griffin from Family Guy) 

Prolonged exposure to evil, despite good intentions. (Frodo from Lord of the Rings, Ice King from Adventure Time)  

Narrow minded destruction in belief of a greater goal. (Harvey Dent from "The Dark Knight Rises")

Now, Macbeth falls into a strange combination of the last two categories. On one hand, he is sort of exposed to evil through the witches and then Lady Macbeth who eventually convince him to kill Duncan. And also he is destroyed because of the belief in a goal (to become king). However Macbeth is better suited in a new forth category which could be described as "Succumbs to temptation of power/money/ect" which would include a ton of modern characters. For Macbeth, that first step was the witches. They gave him the idea that started him on his path to villainy. Macbeth does suffer from these actions, which is seen through the comparison of Macbeth's brain and the castle. The hallucination of the dagger is one example of Macbeth's instability (2.1)

A dagger of the mind, a false creation,

Proceeding from the heat oppressed brain? 

I see thee yet, in form as palpable

As this which now I draw. 

Thou marshall'st me the way that I was going;

And such an instrument I was to use.

and the story about the horse eating the other horse is an example of the chaos inside the castle (2.4).

ROSS
       And Duncan's horses—a thing most strange and certain—
             Beauteous and swift, the minions of their race,
       Turn'd wild in nature, broke their stalls, flung out,
       Contending 'gainst obedience, as they would make
       War with mankind.
       Old Man
                                    'Tis said they eat each other.  

Some other characters who exemplify this quote include:
(Duh, he's the one who said it)

(Because running a meth lab is pretty evil, even if it's to support your family after you get diagnosed with cancer)

(You guys started out being one of the most innovative software companies, but somehow you can't design a phone that has a battery that lasts for more than a year?)

(This one is practically the perfect example:Dean dedicates his whole life to fighting demons only to become one)

(This one doesn't fit any of the categories, which necessitates the creation of a fifth "People who are forced into evil"-and yes that was a bit of a Wicked reference)

(I'll just leave this here because something tells me that everyone will have a different opinion on the problems of the US today)