SecurityFix has a sobering post on systems that are “owned” by gangs in Russia. Here is the M.O. for such gangs:
In DDoS assaults, cyber gangsters demand tens of thousands of dollars in protection money from businesses. If the businesses refuse to pay, the criminals order hundreds or thousands of compromised computers that they control to flood the Web sites with meaningless traffic, crippling the businesses and preventing legitimate visitors from transacting with the sites.
Those same systems can be bought to attack any target.