Might be an interesting argument if death row inmates didn't typically outlive many of their jurors. But seriously? What capital murderer has ever had fewer than three years [minimum] to repent of his crimes?
The Bible doesn't give time limits. As I said, I understand the reasoning, but I don't agree with a total condemnation of executions. From a Christian standpoint, anything involving killing people has to be taken very seriously, because eternal souls are involved.
Okay, you've got a murderer ... he killed X people. Those people can't be brought back; they're either saved or not, based on what they believed and did before their death. However, the killer is still "pending." Is it merciful to keep him "pending"? Maybe it is; maybe he'll repent. On the other hand, maybe he will continue to harm others.