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.

Well the script is there, but maybe someone else put it there, because they were leaving/being forced out and they wanted to sabotage the company and make it look bad for DH (Article says DL).

No need to sabotage the company immediately, revenge is a dish best served cold.

Unlikely, given that he confessed and that in other attack, he damaged the machine only he had access to.

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.

It could even have been another employee putting the idea into his head and encouraging him.

Other employee, reddit story, AI chatbot, crime show... it does not matter who put the idea in the head, all that matters is who did the deed.

Matters to whom