The pussyfooting around by do-gooders , bleeding hearts , in western society is taken as weakness , and only encourages more such like attacks to occurr
The death penalty has never been shown to deter crime and it cost at least 3x incarceration, at least according to every study ever done. Why should other people pay more and get less just so someone can satisfy a bloodlust?
Also, anyone who has ever studied history, read a book by anyone who studied history or seen a movie by someone who read a book by someone who studied history would know that crazy people gonna do crazy things and the death penalty doesn't stop it. Never has, never will.