I think you miss the point. Firstly; the death penalty is the domain of savages, and if you believe otherwise please review the countries which employ it besides the US. Illustrious company you wish to keep. But, politics aside, there's been a theme through the comics which Nolan's films also covered; namely, escalation. I think it's in TDKR where Wolper, on one of the TV segments, suggests Batman creates his enemies - that's perhaps the most extreme version of the theme but it basically says Gotham attracts these criminals to it. If the Joker is killed, there's nothing to suggest that someone else wouldn't fill the role he currently fills. So, a petty, infantile and base urge to get revenge on criminals gets sated yet Gotham is no better off.