I would say that its a battle between good and evil, a battle that has always and will always be part of the scope of humanity. Richard embodies all that is evil, and the good is put down until the final act. Evil exists in humanity, yet someone good has to come along and extinguish it. Look at Richards speech in Act one Scene I, that begins, "And therefore, since I cannot prove a lover,
To entertain these fair well-spoken days,
I am determined to prove a villain
And hate the idle pleasures of these days." That is a true villian of humanity, just as other leaders before and after him have been.