Let me try and be clear
(because sometimes I don't do such a great job of it)
In this situation, a pregnant woman was shot in the head while laying in bed. The person who pulled the trigger either knew it was wrong, or they didn't. Either way, the woman is dead. Either way, the killer has demonstrated that they are too dangerous to be allowed freedom.
Even if "proven" they are "not responsible" for their actions, they should not be allowed freedom, for that reason alone. Jail, mental institution, or execution, I care not, that is for society to decide, via our legal system. What matters is that they never be given the opportunity to repeat their crime.
Not knowing/understanding what you are doing is an explanation. It is not an excuse.
All else being equal (and it almost never is) bigger bullets tend to work better.