it is reasonable to assume that sane mothers don't tend to kill their own children.
it is reasonable to assume that sane people don't tend to commit crime.
There'll be a prize for being able to spot the difference. A booby prize.
The difference is profound and simple to spot, DR, so you should be ashamed of your lack of insight.
It is entirely sane and rational to commit crimes, since there are benefits to be had that are not available to law-abiding people. Such as gaining money quickly, damaging a competitors business, removing romantic competition etc.
A mother who kills a baby? What does she gain? It is all loss. There is a reason that mothers bond so strongly to their babies, and so when that bonding does not protect it then something is very wrong psychologically.
But you could have worked that out, couldn't you?
None of this is an argument for not punishing the crime of murder.
To be guilty of 1st degree murder there must be an
actus reus (a guilty act) and a
mens rea (a guilty mind). If the metal capacity was not there to reach the standard of the mens rea, then they should be aquitted. Society, as determined from case law and statute, prefers to treat mentally ill people rather than punish them.
Are you happy to execute people who are mentally incapable of controlling their conduct?