Execution of innocent people is (and always has been) the entirely predictable, inevitable, and probably unavoidable result of capital punishment. There is no getting around the fact that, as long as the state executes prisoners, innocent people will be executed and “the state”, i.e. taxpayers, will pay more for it than they ever would have imprisoning the convicted for life.
There will ALWAYS be mistakes, bias, and corruption. There is no such thing as incontrovertible evidence. And even if there was some fantastical magical way to know absolute truth, that is still a pretty poor justification for more murder.