A very stupid thing to do.......
When I left an ex-employer a good few years ago, it was 18 months later that I realised I still had cached credentials to the email and hosting setup that I implemented during my time as their IT manager. They were a HORRIBLE employer, and anyone who knows me knows who they are, so I won't mention them here. I could have wreaked similar levels of havoc on their systems very easily, but didn't for a number of reasons:
1) They would know that it was me
2) I would end-up in prison
3) I am a reasonable human-being who bears no ill-will to the other employees at the business
4) Life is too short to bear these sorts of grudges.
5) I quite like the fact that I have moved-on to MUCH better and bigger things since leaving their employment, which would have been impossible to do with a criminal record.
As much as it's tempting (too tempting in the case of this guy) it's just not worth it. As anyone who works in IT knows, we hold the keys to the business, and if we screw-up, accidentally, or like with the above story, on purpose) there is often a lot at stake.