My father was a very gentle man, who suffered from bipolar depression.
On more than one occasion, when seriously ill, he tried to kill my mother and us children. In his case, this was because he felt that our outlook as a family was so awful, and that of the world in general, that it would be better if we all died (him included, once he had 'done his duty').
He was very ill. Not a 'case of the blues', but seriously unwell and delusional. Strange as this may seem, I know that this impulse came from a place of care towards us all. Fortunately my mother knew the signs and was able to intercept his plans and get him the care he needed.
I hate to think what would have happened nowadays- there seems to be a dangerous lack of funding for crisis care.
I agree with a pp: they should either have committed him to hospital (ideally before anyone died), or sent him to jail if he wasn't deemed to be mentally ill.