Abusing hacked computers (not only servers) as spam hosts is very common those days but not the only reason why anyone would hack into a system.
Leaving the system in an unusable state might be caused by some mistakes of the hacker (well maybe it was a script kiddie
).
But those are just speculations only a in depth analysis could bring up the truth. Maybe
http://www.porcupine.org/forensics/tct.html will be helpful...