Because you can’t choose to kill your child.
Maybe I see this differently because I've had to see so much death in my life (murder, cancer, brain tumors, heart attacks) ... the only people left of my family are my sister and I.
There is a difference between "kill" and "murder". There is a difference between "justice" and "mercy". The "just" thing to do may be to provide this child an incubator and keep them alive no matter what pain they endure, or regardless of whether they will ever spend a moment conscious in their entire life because they have a beating heart. The "merciful" thing may have been to let this woman and her child go back to the lord 10 weeks ago before getting to this point, and to let the father and sibling grieve and move on. Sometimes God did what was just, and sometimes He did what was merciful. We can all speculate on what God would do and on what we want to impose on this man and this woman's family. But as a woman who has a relationship with Christ, I can tell you that this isn't what I would want for myself or my child.