I'm pretty sure we couldn't agree on a universal definition that wouldn't be subject to error or interpretation, and after we've killed an innocent person and likely created a martyr and a drive to retribution by some segment of the population, we can't really back out of that mess.

There's a perfectly workable solution that aligns with the imperfections and uncertainties of justice, and conveniently also achieves the same or better metrics as evidenced by countless studies on the topic. Seems like an easy decision.

However that won't satisfy retributive blood lust, or make anyone feel like a tough guy when advocating for it, so the death penalty persists where those things are important.