The way to successfully revenge-hack a company is to delay the attack long enough such that it does not appear to coincide with your being fired.
Create your dummy admin backdoor account well in advance (because you're the kind of bastard that does this sort of thing, you're planning ahead, natch) and make sure there is an obscure VM that you created "for testing" and happened to leave running "on accident."
Then after you are fired, use a free VPN (at a minimum, TOR would be better) from an Internet cafe (pay in cash) in another city; remote in to the VM and setup a self-deleting Scheduled Task on the server to nuke the database or bork the backups (or whatever havoc you wish to cause.) But the key thing is to set the start date/time for the malware launch in the future by at least 60 days. Then delete the VM and the back-door account and wait for your glorious revenge to serve itself.
The other key thing to do is to NOT act all crazy when you're let go - be upset (you *are* being fired after all) but not angry so they won't immediately suspect you're an asshole.