If they got along ok for a few months - I wonder if Zorro is unwell, perhaps feeling pain from some unknown problem, and taking it out on Meg?

He might have a urinary infection or something else. Have you had Zorro checked out lately?

With a sudden change in behaviour, my first thought is health problem.

All the furkids are spayed and neutered, right?

A thorough checkup at the vet for Zorro, including urine, would be my suggestion.