Against, unless its an extreme case of mass murders with no regrets.
For example the shooter in sweden. He probably should be executed. It's people who have psychological defects that are so strong that they are a danger to themselves and others. At that point, its best for everyone to put them out of their misery.
Though i can understand the case of murder without regret aswell. It definitly makes me think. We must protect life and taking someone's life should come at an equal price. The problem then is that we become as bad as they are.