"Why in god's name would they let him live, and eventually let him free, for what he did?"
Because draconian sentences fosters more crime, not less.
With capital punishment on the agenda, and liberally applied, there's no reason for a criminal to not kill any innocent bystander and law enforcement official even remotely close to identifying or capturing them. After all, it's not as they'd be risking anything more by doing so.
Putting people, violent criminals or not, in a situation where they have nothing to lose is a _very_ poor idea.
Your opinion does shed light on one of the more common misconceptions of our time however, that of how people judge the justice system. The point isn't to punish criminals, it's to prevent further crimes from being committed.
The purpose of the justice system is not to appease victims, or families of victims, by some medieval measure of eye-for-an-eye punishment. Rather it's tasked to protect the general populace from crime and abuse to the extent possible without infringing on the liberties and freedoms of said populace.
Sometimes that entails locking people up for a long time, though the reason for doing so is protecting the rest of society - not punishing the criminal.