Although I'm very much against the death penalty and I am very much proud to live in a country where it's abolished, I'm really bothered by the attention it's given in light of its lack of an affect on deterrence of crime:
(http://www.amnesty.ca/deathpenalty/canada.php)
"Contrary to predictions by death penalty supporters, the homicide rate in Canada did not increase after abolition in 1976. In fact, the Canadian murder rate declined slightly the following year (from 2.8 per 100,000 to 2.7). Over the next 20 years the homicide rate fluctuated (between 2.2 and 2.8 per 100,000), but the general trend was clearly downwards. It reached a 30-year low in 1995 (1.98) -- the fourth consecutive year-to-year decrease and a full one-third lower than in the year before abolition. In 1998, the homicide rate dipped below 1.9 per 100,000, the lowest rate since the 1960s."
My main concern is one that is often not given much serious regard and is often overlooked in these discussions on crime, as it doesn't act as an immediate deterrant to current criminals - prevention. As unqualified as I am to say, this particular heinous act seems to be unpreventable. There are undoubtedly sick people in this world who are likely to commit abominable crimes regardless of the punishment, whether it'd be death or life imprisonment. But it's also likely that there are even more who commit these crimes and are very much preventable - no source, but I would think it obvious.
I think that alone is reason enough to end the death penalty (in whichever countries it is still used), simply to move those resources to prevention. It's been evidenced to work in Canada (though we still have a long way to go as gang violence is becoming a major issue).
As opposed to what username2324 suggested, there are several reasons to oppose the death penalty outside of simply clinging to 'life is precious', most of which I agree with but have already been explained by others on here. I won't bother to dwell on any further.
