[Kleinzeit]

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.

Or... They knew it was wrong legally but believed it was right morally, because of some other factor that we currently know nothing about. (I'm not saying this is likely, I'm just saying it is possible. The fact that she was pregnant doesn't make it any less likely that she did something to contribute to this. And no, I'm not blaming the victim. I'm just pointing out that we are making judgments based on very little knowledge of the situation.)

Either way, the killer has demonstrated that they are too dangerous to be allowed freedom.

Yes. For the time being. But we don't know why that person became dangerous. It may have been in response to an extreme situation that is now over. We are in no position to decide that they've demonstrated they will be too dangerous forever.

Even if "proven" they are "not responsible" for their actions, they should not be allowed freedom, for that reason alone. ... What matters is that they never be given the opportunity to repeat their crime.

Agreed. Abolutely. And more than that, what matters also is that they be punished for the crime they have already committed. But "not giving them the opportunity to repeat their crime" is relative; if there is no sign that they ever will repeat the crime, and there is every sign that they will not, then you don't have to kill them or lock them up forever (at huge expense to the taxpayer) to guard against that possibility.

Not knowing/understanding what you are doing is an explanation. It is not an excuse.

Again, I agree. But imagine for a second this scenario... A guy's wife leaves him, he has a few drinks, he has an argument with a friend, he takes some sleeping pills because he can't sleep, he wakes up in the middle of the night, heart pounding in his ears, adrenaline coursing, fumbling in the dark because he heard a sound he doesn't understand, and he fires a shot... That person is really going to be hoping that others in his community will be willing to consider mitigating circumstances when the wheels of justice start grinding over him.

You know that saying, "There but for the grace of God go I"? I understand your frustration, but mercy is a powerful thing. We will all, at some time, be hoping for it from others.