Norway should've put that guy away for life alyster. The mistake there was the decision to only lock the guy up for 21 years. Honestly, is there a significant difference to the outside world if an individual gets put away for life without parole or if they get executed?
To avoid the risk of accidentally executing an innocent person (let's not forget several times people have exonerated days if not hours before they were supposed to be executed), just lock the person away for life without parole.