I know what he did was completely wrong and I know that it hurt people. And yet a small part of me cannot help but admire the genius it would take to pull off (well... obviously he didn't completely "pull it off" or he wouldn't be in jail right now) such a LARGE investment scam. Especially when your "company" was actually investing in NOTHING.
But, here again we see the major downfall of nearly every genius who sets their faculties to completely selfish ill-gotten gain: greed. Just think: had Madoff said, "Enough already, I'm done. I'm going to take my billions to a Caribbean tax haven and spent the rest of my days living a luxurious life on the beach," he would have ACTUALLY "pulled off" what would have been the biggest scam in history. But he didn't. He kept going. And he got caught.
The same as the bank robber who gets away with a huge robbery... but then tries again. Or the serial killer or rapist who "gets away" with their crimes... but continues to commit them until they are caught.
Perhaps it is the unquenchable drive for money, control, or power that drives them to commit crimes in the first place in order to satisfy these desires that ultimately results in their seeming inability to "quit while they're ahead."