Joss Whedon has the most persuasive and stunning vision of evil in all of popular culture. To begin with, Whedon's evil is a positive evil, by which I mean that it really exists in-and-of-itself rather than being merely the absence of good, as it is in Dante (where Hell is what you get when you can no longer apprehend God). It is also not at all like evil in C. S. Lewis's Christianity, where Satan merely thinks he is evil; in the grand plan of God's benevolent design Satan's rebellion and temptation of man has its part to play, and thus, in the long run, his "evil" is in the service of good. For Whedon evil is pervasive and fundamental: as Holland Manners tells Angel, the world runs because of evil, not in spite of it. As Buffy and Angel end our heroes face primordially evil antagonists that cannot be destroyed (though their representatives can be harmed): the First Evil and the Senior Partners live forever, though Caleb, the Uber-Vampires, and the Black Thorn are killed. And though there is lip service to the powers of good, it is ultimately empty: the ancient women who watched the watchers are useless and tacked on in the second to last episode of Buffy. On Angel The Powers That Be are distant and weird, communicating through cold and dangerous representatives, and the terrifying Jasmine reveals herself to be one of them. Angel makes clear that in resisting evil we define ourselves, which is good for us, but has nothing to do with the cosmos. In order to fight evil, Whedon's characters, who don't call on the powers of good because there are none, harness evil to fight evil while trying not to lose their humanity: Slayer powers are essentially demon powers and, in season seven, Buffy rejects more power because it means less humanity; and Angel, of course, is a vampire with a soul, who, in Angel season one, refuses to become human because it means he won't be able to oppose evil with the same force. The soul, in Whedon, is so poorly defined as to be essentially meaningless (though on the surface level it makes for great drama): characters with souls do evil (Buffy's dying childhood friend from "Lie to Me"), and characters without souls do good (Spike). Evil, on the other hand, is defined clearly and extensively over seven seasons of Buffy and five seasons of Angel.
What I find less persuasive in the Buffy and Angel universes however -- though it doesn't really bother me as ultimately it plays little part in most of the actual episodes -- is Buffy and Angel's desire to be merely human: Buffy wants nothing more than to live life as a normal girl free of Slayer responsibilities and Angel's fondest wish (though he signs it away when he thinks it no longer matters) is to fulfil the prophecy in which the vampire with a soul is granted his humanity (Angel and Spike even go so far as to fight over this prize). Though Spike comes back as a network demand, notice the two characters that don't make it though the Buffy series finale are both not human. Whedon, of course, draws tremendous power in injecting a vital, human element into fantasy and horror genres (and the sci-fi and western in Firefly and Serenity); superhero comics, however, have made strides in thinking through the consequences of post-humanism (as I discuss in my essay on the X-Men here). It will be interesting to see if Whedon's future work in the superhero genre (Astonishing X-Men, Wonder Woman) will ever be effected by this. But as long as he remains a genius, it won't really matter -- he is, after all a master storyteller rather than an master philosopher, as he himself admits on Firefly's "Objects in Space" commentary (though without the word "master," obviously). Long Live Joss Whedon, is my thinking.