Plus the question doesn't make sense - if you're opposed to the death penalty, you're opposed to it in all situations. And I hate to say it, but this isn't even the worst or most senseless and brutal of crimes possible.
A trial isn't fool-proof. There are so many factors and variables that influence the outcome. We try to make it as fail-safe as possible, but it is a human process run by humans. Even the appeal process doesn't necessarily cure all defects.
Get used to me. Black, confident, cocky; my name, not yours; my religion, not yours; my goals, my own; get used to me. -Muhammad Ali