The LSE's European Politics and Policy blog has published a short piece I wrote on the Breivik judgment. Here's the conclusion, summarizing the way in which opposition to capital punishment gradually became the majority position in Western Europe:
This pattern can be summarized as follows. After a period of active controversy, interest in the subject of capital punishment fades. Since inertia (always a powerful force in politics) now favours abolition, there seems little point re-opening the emotional debate over executions. No drastic increase in violent crime will occur after abolition. If there is a crime increase, the experts will reassure the public that abolition had nothing to do with it, as the death penalty has no proven deterrent effect. Eventually, the press loses interest in the subject of capital punishment’s potential return, and politicians realize it has lost its power as a vote-getter. Open support for capital punishment lives on only among right-wing fringe parties (such as the British National Party, or Germany’s far-right National Democratic Party). The adoption of capital punishment by fringe parties therefore creates a sort of self-reinforcing ring-fence around the issue: even mainstream politicians who might personally favor the death penalty choose not to mention it, for fear of being associated with unpopular fringe groups.
At the end of this process (which can take decades) we are left with perhaps 60-70 per cent of a country’s population opposing capital punishment in principle. Notably, this opposition may be relatively weak — as I found while researching my book on this subject, leading questions can elicit support for capital punishment even among people who consider themselves abolitionist. This is where treaties come into play. Even if shocking crimes such as Breivik’s might prompt some citizens to re-think their death penalty views, international commitments authoritatively banning executions stand in the way. It’s difficult to build support for a policy that has no chance of being adopted.