Life in Prison with Parole after 20 years (which should be the maximum existing sentence IMO).
The guy is committed to killing people. I don't support the death penalty, but releasing him is just as insane as killing him in my view.
If he hasn't reformed after 20 years, then he won't be released. But every person, even the most supposedly "evil", can change, and should be given a fair chance to prove they have reformed.
I agree in >99% of cases, but after a certain point I don't believe it matters whether the person has reformed or not. There are certain crimes that simply merit actual life long punishment. I honestly restrict this to cases like Breivik or Tsarnaev, but I see no reason to care whether they've changed or not.