Why do people hack computers? Motivations have changed from exploring the tech landscape (1990s) to fame in the community - "leet" status (early 2000s) to filthy lucre (today).
Cisco has a very interesting paper by their security research team, where they tracked down a "bot herder" (someone who takes over computers) and spent some time with him in IRC.
If you're interested in the psychological - rather than technical - aspects of the Dark Side, this is a very good read indeed.
ObDisclaimer: I used to work at Cisco, and know the folks involved in this report. Smart guys.