I originally upvoted you, but then I took a moment to read your comment more closely.
It needs to be reserved for the most monstrous of crimes and, in this case, 5 undisputed murders plus possibly 5 more?
No. Being convicted of 5 murders does not mean 5 undisputed murders. The primary argument against the death penalty is based on how often we incarcerate innocent or even insufficiently proven guilty people and then the only recourse is letting them out, since we have no recourse after wrongfully executing someone. Portraying the 5 murders he was convicted of as “undisputed” wilfully ignores the single biggest problem with the death penalty.