I like his message mainly because I agree with the death penalty in principle. If you want to be a shitty human being, with 7B people on the planet we don't really need you. But, in practice, we will never get it 100% right. Probability of killing an innocent person will always be >0. For that reason alone, we should get rid of it.
The importance of that I think is clear with respect to the dynamics for terrorism and war. People who experience their loved ones dying easily get vengeful and radicalized.
If I lost somebody dear to me, I don't think the execution of the killer that my increased satisfaction from this would likely pale compared to the sadness inflicted on those who cared about the killer.
This is a fundamental thing in human psychology. We react more to losses than gains. If e.g. the boss takes money from you and gives to me as a bonus, then my joy from this will be significantly less, than the dissatisfaction you will feel from this loss. The sum is lesser than its parts so to speak ;-)