That's the most important thing to consider when conversations go this way. Ultimately, who is to decide what deserves death?
My answer is raising kids better, and socializing them better. The cure is to kill people who behave "improperly", the prevention is making sure children are raised in such a way that they're not exposed to dangerous elements likely to **** them up for the rest of their lives.
Statistically the majority of anti-social behaviour stems from events/trends in childhood. Instead of putting people like Harris and Klebold to death (Hypothetically, had they survived) you need to ask what preventative measures could have been taken so that the abuse they took at home/school could have been eliminated.