What motivates Macbeth to take the evil path he chooses.

I would argue that it is a combination of things that does this.

First, I would start with the idea that Macbeth is ambitious.  If he were not ambitious, nothing else would have mattered.  However, his ambitions did not send him along the path to evil without help.  Instead:

Second, the witches' prophecy gave him reason to believe that his ambitions could be realized.  Before that, he had ambitions, but they were under the surface. Now, he sees an opportunity and that makes his ambition wake up, as it were.

Third, his wife pushes him.  She's ambitious too and she thinks he is not ruthless enough.

So it's a combination of those three.