Death sentence

How come everybody made a fuss about an iranian woman convicted with paying a guy to murder her husband and sentenced to be put to death, and nobody raised a voiced about the american mentally retarded Lewis who was put to death for paying 2 guys to murder her husband ?
My opinion about death penalty is : does it lead to anything positive and constructive ? As for psychopaths, serial killers, sex offeders, death penalty might prevent other murders to happen and thus protect innocent people. The major problem remains : is this convicted person really and surely the murderer, because if it turns out to be another one, we would not be able to ressucitate him and make up for the judicial mistake, so may be it's better to avoid killing innocents and go for life in prison for serious killers. Another problem is : is this killer really responsible for his acts or does he suffer from abnormal genes and chromosomes that alter his behaviour , such abnormalities that science still does not aprove as diseases but might do so in the future. In this case, we might consider death penalty as some kind of natural selection to get rid of bad genes, but is it ethically humanly acceptable? People might say death penalty is dissuasive for other criminals, but data from countries that applicate it does not seem to show decrease in crime more than in other countries that ban it.
I think death penalty is the ultimate kind of revenge to wipe the victim's blood in the eyes of families and friends. Since we cannot ressucitate the dead, revenge is the only option to feel that justice is done.