This is premeditated murder in the first degree, given how artful the employer was in blackmailing them and getting the surviving maid to cover up her abuse before the authorities.No insane person can be as cunning hence an insanity plea should not suffice.
And yes the authorities themselves have blood on their hands for not taking enough measures in protecting the oppressed employees.
Back in Sri Lanka, there are intensive training sessions for all those oversees bound maids. Given the sickening rise in killings and torture these poor women must endure, I'd say they better start including martial arts and other self defense type courses to the list along with how best to serve tea!!