I'm gonna have to go against the chorus and lay the blame at your feet, honestly. You left a booby trap for whoever rebooted that server at any point in the future. Had you removed the hard drive or put the machine into a donation pile, I could understand.
Say you get hit by a bus the next week and they hire a new sysadmin. A few days later he's asked to setup a new service and decides to repurpose that unused server. He connects it to the network, boots it, installs updates and new software . . . and then gets pulled onto some other task that takes a day. That night disaster strikes. Is it his fault for not ensuring there were no dangerous cron jobs left on the machine?
Perhaps, but it's much easier to disarm bombs you've designed rather than force the job onto some poor, unsuspecting sap.