Amnesty International, UK is currently focusing on the death penalty. They've got a column in the London Times on it right now. Some points in it are valid, some are less compelling.
There's a few different arguments people have about the morality of judicial killing. Some, as the author notes, are pragmatic: what if you've got the wrong person? As he notes, DNA evidence has a context in which it was collected and can be contaminated or inapplicable. There's also the contexts around the trials themselves, like, are the lawyers sober and awake? And then there's the way that class and race play into trials. People of color are way more likely to get death sentences. And people who have public defenders. And people who have incompetent lawyers. If the death sentence was fairly applied, it would fall with equal likely hood on the rich and the poor, the white and the non-white, etc. It doesn't and this seems to imply a system where people who are sentences to death are more likely to be people who might not have even been found guilty if they had proper legal representation and a anti-racist jury.
I had an argument with a friend several months ago about the death penalty. As it happened, we had both just heard of the Birmingham pub bombings. This was the most deadly bomb attack from the IRA. My friend argued that certainly that case was one that would have deserved the death penalty. Indeed, the sentencing judge agreed and lamented that he couldn't administer it. However, later it turned out that the police more or less randomly grabbed a group of Irishmen and tried them. Because the IRA, and by extension all of the Irish, were guilty by nature of all being the same alien other. The actual guilt or innocence of individuals is less important than stamping down on the other as a whole. And the way to stamp down is to give the most harsh penalty we can administer.
I don't happen to think it's moral to kill people (except in self-defense) and I'm not keen on it being done in my name, and that's why I'm against the death penalty. If you think killing people is ok, then you've got to be ok with the amount of error which will creep into any system and especially something as fraught with error as the court system. What percentage of "oops, wrong guy" are you willing to tolerate? Because a perfect system is impossible, there will always be some percentage. How much is too much?
And this leads to the strongest pragmatic argument against the death penalty. Do you trust your government with the right to kill you? Maybe you're not poor or a person of color or otherwise at exceptionally high risk for being accused in error, but it could happen. Would you trust the justice system with your life?