What an idiot.
Should have left database triggers and cron jobs that run update scripts with innocent sounding names that randomly (but with a gradually increasing frequency and severity) make slight changes to the data that accumulate over months to corrupt the database, leave no way of figuring out what data is correct and what isn't, and which renders backups useless because they all contain variations of the corrupt data.
Phone numbers and email addresses with transposed digits, swapping customer order records with each other, altering dates, receivable and payable amounts, stock levels and locations, compliance data that'll attract regulatory penalties if wrong, getting worse and worse until it's inevitably discovered and no longer attributable to human error, but far too late to do anything about because like a cancer it's spread -everywhere-.
If they dare fire you, burn them to the ground slowly. Bonus if you can pin the blame on another employee, and end up getting called back to consult about fixing things ;D
C'mon, if you're going down the route of being a sociopathic sysadmin, do it right.