In Canada, in the last decade, as DNA testing has become available, there have been quite a few high profile cases of people serving life sentences for rape and murder, most notably David Milgaard, who have had their sentences overturned.  They were wrongfully convicted and, if this were the U.S., could have been executed.  The end of the death sentence in Canada has not led to a crime spree and, on the contrary, has made it possible to overturn wrongful convictions.

The other thing that convinced me was the sociological data on who gets executed and who gets a lesser sentence for the same crime.  The colour of your skin does make a significant difference.