Since defilers in the original DARK DUN setting are always neutral or evil, they're pretty much all selfish assholes, as the people at large believe. Naturally, the dragon kings are also defilers, so that's not helping anyone's reputation any. Of course, defilers are selfish assholes for a reason -- ultimate personal power. A defiler gains levels and additional magical abilities slightly faster than a preserver (the "typical" mage variant in DARK SUN, who isn't wrecking the environment by draining the life out of everything in sight). This makes them great villains; your DARK SUN players can fight defilers at any level, because they're pretty much always bad dudes and you can feel a nice sense of self-righteous eco-preservation when you give one a justifiable beat-down.
As always with villains, the storied DM will change up this formula from time to time. Sticking the party with a defiler with whom they have to work for some shared goal is a great way to ratchet up tension (assuming that the party's druid doesn't eviscerate the defiler regardless). Preservers can still learn spells by stealing them from defiler caches, and that makes a wonderful hook too; if the party can't figure out the defiler's magical code, the party's mage may have to cut a deal in order to learn much-needed magical secrets from the jerk who doesn't care about the Tragedy of the Arcane Commons.
The juiciest bit about defilers, though, is temptation. For some DMs, defilers are just bad guys who wreck stuff and that's that -- no complexity; we know they're selfishly destroying the world, one 10'-diameter circle at a time, and there's no moral question about where they stand. But the defiler does have advantages over the preserver. Their magic is generally more powerful. They can prepare and deploy more spells. Their magic, even the otherwise inoffensive spells, draws on life-force so painfully that it can shock and slow their enemies.
The good DM knows that for temptation to work, you have to offer the players something that they actually, legitimately want. The temptation must be something that makes the player agonize over the choice; otherwise, it's not tempting! When a demonic creature offers the player a magical sword in exchange for baby-murdering, nobody's going to take the deal; the sword's obviously not worth the price, and it's probably cursed, too. The temptation of defiling, though . . . you can have just enough power to make a difference. You can save your friends' lives. You can fight the templars of the sorcerer-kings and defeat them. You can reshape the world according to your whims! All it costs is the lives of a few lousy plants and trees. Who cares about that? It's just a tiny little sacrifice, right? And it doesn't cost you anything. And what you get in return is pure power!
That's where the defiler shines in a DARK SUN game -- not just as a villain, but as a temptation. Every PC mage is someone who could be a defiler. The angry mobs who lynch mages aren't wrong: Somewhere in every mage is the tipping point that would push them over the edge into defiling. Wrenching that magical item from the templars . . . freeing a slave . . . saving your wife . . . there's something that will make the mage say "This time it's worth it. Just this once."
But it's never "just this once."
And that's why the world is a bad place. That's why we can't have nice things on Athas. Because the people who can make the biggest changes, the people who wield the most potentially wide-ranging powers, are the ones who have the greatest temptation to use that power for destructive ends. You can bring about a new age on Athas. You can bring safety and security to the helpless people of the land. You can overthrow the sorcerer-kings. All you have to do is become one of them.
So, are defilers evil? Of course they are. They're the kind of evil summed up by that most apt description: "All it takes to make a villain is to put a reasonable person in an unreasonable situation." Well, Athas is full of unreasonable situations.
Can you play a mage who never succumbs to temptation? Who doesn't bow to blackmail, extortion, loss, hate, revenge, any of those horrible things that can and do happen all the time to the residents of the burnt world? There's only one way to find out . . .