Well, if that were true, there wouldn't be murder. What is true, though, is that innocent people have been capitally punished, and there's no undoing that.
Discussion
Infantile argument
Power tends to corrupt, and absolute power corrupts absolutely" (Acton).
I would trust street vigilance before I'd trust the government with capital punishment.
If you read the thread you would notice I prefaced my entire argument on the restoration of jury trial as defined by the Magna Carta