I personally beleive the death penalty shouldn't exist for people like Brevik. The best case scenario is he suffers for the rest of his days. He can't be rehabilitated and he shouldn't be.
His acts were so evil he deserves to suffer. Laws should change to ensure people like him can suffer.
Suffering isn't pointless. It makes me and many others feel good if horrible people like Brevik suffer, its justice for the people who suffer everyday due to Breviks crimes. Governments through history have been responsible for a ton of suffering - this is one of those rare instances where it's fully justified.
But I'll admit, wanting revenge isn't great and you're right, we shouldn't setup systems to enact revenge. That being said, some people are so terrible they need to be locked up and separated from society permanently. Since enacting a death penalty is both expensive and takes a very long time if you want to have the proper checks and balances to ensure innocent people aren't being executed, that tells me life imprisonment is the only option for people that would or should be otherwise executed.
Maybe he changes his tune in 10 years and becomes a voice against the terrorism he tried to inspire. That could end up with an at least beneficial outcome for society.