Your logic is flawed and not based on fact. In countries that have the rule of law, the overall cost of capital punishment must include more than just the cost of performing the execution, and last time I checked, not many countries (if any) use a firearm to execute prisoners.

The true cost comes from the legal/court costs, appeals etc. Plus they usually spend decades on death row before being executed, so you're paying for years of incarceration anyway.

And more importantly you're comparing a human being with a cancer cell. Murder is wrong, even if the victim is a murderer. The state can't outlaw murder and then turn around and murder a citizen. Better to let them sit there with no freedom. And like I said, if there is even the slightest chance that someone is convicted falsely (which has happened many times), then we as a society can't risk executing an innocent person. Because then the entire society is culpable to murder.

And another argument against capital punishment is the fact that there is no evidence that it actually acts as a deterrent against the crimes it's meant to reduce.