There was much more to this story than posted, she was vaginally and anally raped and did require surgery to repair torn tissue, and although that is terrible to even consider as is, she is still alive, which is one of the things the U.S. Supreme Court said during the discussions on this was that if the death penality is going to be imposed for child rape and for murder, these offenders may not be willing to take their chances on their manipulation tactics and go ahead and murder them because the sentence is going to be the same if caught anyway. Of course this is based on presumption by the court and many may agree or disagree.
^^ I hope I worded that right, but if not, you can read it for yourself.
http://www.scotusblog.com/wp/wp-content/uploads/2008/06/07-343.pdf
You also have to take into consideration the emotional aspects the child victim would have to contend with if they also knew that the offender was executed for the crime(s) against them. If it is a stranger, that isn't too difficult, but in this case, this young girl had an established bond with the offender; she was his step daughter.
Studies have shown, and of course this is based on the recidivism rate of these offenders that sexually assault a stranger victim (one known less than 24 hours) is at high risk to offend again. Those that sexually assault a known victim, one whom they have had a relationship with over time, are least likely.
The problem I have here, although I do agree with the U.S. Supreme Court on this particular case, is that it puts a blanket ruling out there for ALL THE STATES and they have now lost the option of going on a case by case bases.