He literally wrote a script that locked out everyone on the network the second they disabled his employee account in active directory. He even named the variable Is[DH]EnabledInAD where [DH] is his initials.
No need to sabotage the company immediately, revenge is a dish best served cold.
I mean, it is theoretically possible there were two disgruntled employees, and second one pinned his crimes to the first one, but I think this is pretty unlikely, given the observeed lack of common sense.