The decision to end their life is something one can understand..after all, diagnosed pre birth it would have been an option on the table up to the day before the baby is born. As a society we would today accept a decision to terminate the foetus prenatally with this diagnosis....so why not after birth let parents make that decision ?
However, if this was the decision it should be made with the father too. And it is illegal.... We don't know if he was aware.... And what abput the impact on the other child? any preparation ? one imagines not...(unless it was a pact? )
My ex spoke many times our son was "better off dead" (sld/asd) but I had no idea he might actually follow Thru in actions until the day he picked him up and threw him across the room screaming why are you disabled I don't want a disabled child... fortunately I was there... I attacked my ex to stop him... Son was ok. it was damned scary.... He my ex has episodic severe mh issues.
(Ex has since had therapy and now few years in expesses diff views re son...I semi trust him..)
Shortly after that incident when ex was away getting help, I found out that someone I had met online with a son with much more severe issues had had her son killed on an access visit to her ex. She knew her ex had had depression etc but had no idea or inkling he had thoughts to kill... She thought her ex loved him...
Nor did I . I was lucky. I caught my ex In The act of harming.... Perhaps he meant to do it this way i dont know... This lady did not. Her ex gassed himself and child.. She was devastated. Her child also had a condition which was life limiting but she did not share the views of her ex about his life. She did not want her ex deciding when and how he should die... Without any forewarning.
I think it is unfair on the other parent to make that decision alone.
But we don't know here what the situation was, we dont know if this was discussed we dont know how much psychiatric or emotional support they had how many discussion the mother and father had.... Practical help she had access to.
I have also met a baby with Sma I who died at eight months and I have an inkling how hard it must be...insofar as I can... To have three children growing up to die... V hard.
But if this was a decision made alone then it was unfair on other family members...
Tragic for sure.