The issue isn't about finding if some specific person deserves the death penalty or not. There are people, like Anders Breivik, where their guilt is pretty dang obvious.

But can you come up with some standard of evidence that would be 100% accurate in all cases? Real life tends to get complicated very quickly, and there have been people that have been executed on seemingly clear-cut evidence, only to be exonerated later (Timothy Evans comes to mind). When the cost of failure is so high, it's a safer move to just avoid the risk altogether.