Those kind of questions never occur in a vacuum, so there's no simple answer. What were the circumstances of this magic one-time murder? At least you'd expect a pretty thorough evaluation and monitoring of the person, probably while they were detained.
But anyway, there's still the question of deterrence, which is not the same as retribution. If others see you can murder with no penalty, that may increase crime.
> What were the circumstances of this magic one-time murder?
There's many cases of say, a father going out and murdering their daughter's rapist. It's unlikely that they would kill people that aren't raping their daughters, but if you simply let them go free, it really kind of signals that murder is okay as long as you can justify it well enough. Generally the only time we really condone killing someone is when he's defending himself... and sometimes not even then.
I do think that putting said person in prison for a period of time is appropriate. No punishment whatsoever is condoning murder.
It's just that it's tough to condemn decent people like that to a prison system that is so focused on being terrible for everyone in there. I think prisons need to be a better place to live for all prisoners and the current system simply isn't adequate or reasonable.
But anyway, there's still the question of deterrence, which is not the same as retribution. If others see you can murder with no penalty, that may increase crime.