He may indeed have been a killer. On the other hand he may not have been. A fair appeals system would have overturned what was pretty clearly an unsafe conviction.

Before being killed he served over 20 years in jail. That period means that the man who was killed was essentially a different man from the one who may or may not have done the earlier killing.

The bizarre thing is that this combination of a life sentence with execution to follow is evidently not seen as a "cruel and unusual punishment" in the USA. Unusual? I don't think there is anywhere else on the planet where this could happen.