Theories on why people are attacking L4D2 servers?

What reason do you guys think this person or group(s) has for doing this?

Is it because they just want to see the world burn, trying to advertise/sell their security exploits, testing out their ddos attacks, or redirect people to malicious stuff? Are they trying to improve their capabilities for further attacks on Valve servers and more?

I know that TF2 has or had the bot problem too. I play mostly local servers so this doesn't really affect me, but I'm curious on what you guys think.