I would say that ulimately this is a good idea. Not killing criminals specifically to acquire organs, but consider the following:
-I am 99.9999% sure the crime the person comit to have the death penalty sentenced to him was a Felony. And probably one of the most heinous acts at that.
-Once commiting such a harsh felony, we lose many of our rights and liberties, such as the ability to vote
Wouldn't it make sense that the right to have a say in what happens to your body when you die could also be lost? This act could be just the criminals final act of repayment to the community for the acts they did.
Keep in mind: These people didn't steal a candy bar. They were mass murderers, rapists, abusers, and evils you can't possibly imagine. They aren't going to use their organs after they are dead either way. Part of the sentence could force the offender to be an organ donor. Killed many lives? Well: You're going to potentially save a few as part of your payment back to the community.