For some reason it wouldn't let me contribute my comment to Eva's blog.
(to follow on from Ben's comment:)
I also suggest that people could create viruses for the simple joy of making something work. A varient of this is explored in xkcd: http://xkcd.com/350/
Engineering a virus is similar to making a model aeroplane, or a replica combustion engine. Kids like to play through (among other things) making stuff that works: cooking; playing with K'nex etc. This is popular with kids as it requires discipline (following the recipe/instruction) and there is a positive reward at the end (a delicious meal or a working crane to play with.) also there is a physical output that can be rated by other people.
Some of these kids are now grown up. I kind of agree with Ben and Oliver; creating a virus in most cases isn't for any real monetary gain or anything else but a hobby, making something inanimate that works. Perhaps a nod to/from an innate desire to produce tools and technology?
It's interesting that the things we create have to be destructive. Almost as if we are predisposed towards destruction and carnage. If I see a frozen puddle relfecting the light in an interestingly beautiful way then I have to hold myself back from instantly jumping on it. Maybe it's jealousy? Perhaps it's because the created viruses only attack virtual objects, that aren't physically real and so are perceived to have less value? Maybe the anonymity of the internet allows us to live out our destructive desires without fear of retribution.
Maybe we're constantly frustrated by all the order and secretly long to introduce a little chaos to the pattern, like the Joker in Batman.