I find it helpful to remember the context in which the ancient Hebrews received the Law -- they were a nomadic people, without the capacity for lifelong incarceration that is now available to most modern peoples. In that society, the options for criminal justice were different from the options we have today.
To my mind, of course God was justified in ordaining the death penalty as part of the Law. However, that does not mean that we are required to exercise it today, nor that it is the best option for our current society. We can imprison people in a way which was impossible in an ancient nomadic society.
We have the ability to offer a measure of grace to someone who deserves death; why would we, who have ourselves received grace, seek the death penalty instead of life imprisonment? Shall we not use our freedom to choose a measure of mercy, sparing the convict a most extreme punishment which, while not forbidden in scripture, certainly is not required?