I can't say I didn't fantasize about throwing a supermagnet into the data center of an ex-employer I was downsized from, but I knew better and the majority of adults I hope would know better too.
Yeah, I have had those fantasies too. You don't realize just how much damage you can do until you sit and think about it. After being let go by a retail chain with about 700 stores I realized that in about 15min I could pretty much put the entire chain out of business. They had just scrapped all their phones for VOIP and I had the passwords to all the routers and knew they had the domain admin password hardcoded into the mainframe (I had tried, unsuccessfully for over a year to get them to change that).
It would have been very easy to vpn in using the admin account, telnet to the furthest VOIP router and erase mem my way back to the office effectively wiping out their phone network. Then, set the tape robot to bulk erase (they didn't use offsite storage - too expensive), wipe the fileserver, domain controllers, AIX and Linux and logoff. They also refused to buy any intrusion detection software so very little chance of getting caught. I would never do that, but it gave me a little perverse pleasure knowing that I could.
That company is still in business and I know of at least two instances where they have had breaches due to their refusal to implement even the most basic of security precautions. Still, I should thank them for canning me. I now work for a Fortune 500 company making more than double what they were paying, so all's well that ends well I guess.