Americans love to piss and moan about all the freedoms we supposedly have, but are conspicuously unsatisfied with merely removing said freedoms as punishment for crimes.
Maybe they don't think "freedom" is that important or meaningful
Lets see how true this really is: https://news.ycombinator.com/item?id=39826176 - "Julian Assange granted permission to appeal against extradition to US" - https://www.theguardian.com/media/2024/mar/26/julian-assange...
In principle I have no trouble just executing Breivik; his guilt is established beyond doubt, he committed an act of exceptional evil, and he more or less declares to want to do it again (well technically he says he wants to be a "non-violent Nazi" or some such, but that's a contradiction in terms: "oh that Holocaust thing was just brilliant, more of that!" is violent rhetoric).
But Breivik-type case are rare. So rare it's not really worth changing the system over it. There's principle of a thing and the practicalities of it: in principle the death penalty is fine, but practically organizing that in a legal system with zero false positives is very difficult, so it's not really worth it.
Aside from the US, you can also look at post-second world war in Europe, which saw some executions that were rather over the top in hindsight.
If you spend some time in the Nordics, you'd find that most don't think "death penalty is fine in principle" as it goes against many of the principles people there try to live by.
Other than that killing him would be way to kind.