Another way to frame it:

Assume the justice system is perfect and only guilty people are executed. However, by law for every five or ten guilty executions, a random innocent civilian is also executed.

That system is obviously abhorrent and unjust. However, that's how the system works right now. For every N truly guilty people executed, there's a truly innocent person executed. The only difference is we justify it by calling that person guilty even if they aren't.