I think my revised opinion is that if you intentionally murder someone without a damn good reason, you should spend life in prison. This can include second degree murder, which is often not life in prison in the US (And also happens to line up with the OP scenario).
A lot of conversation needs to happen on this, and we need to constantly reevaluate our positions.
We can't make the punishment the same, or he would just murder her, yet I don't think such a person should every be let out.
Murder is the "easiest" crime to use to make my arguments. And even then I have to specify stuff like "intentional without a damn good reason".
Luckily many places around the world solve it differently so we can look at those systems and observe.
Do you believe people can change? Or perhaps more generally, what do you think the purpose of a prison should be when considering rehabilitation vs punishment?