Why is the application of the death penalty so arbitrary?

I think we need to put another moratorium on the death penalty until we find out why the death penalty is applied so arbitrarily, why many executions are found to be wrongful, and more minority offenders are executed than white offenders, despite a similar ratio of crimes. There is no easy answer and we need to be prepared to stop executions for years, perhaps decades, while the penal system is revamped.