The Courts are right on.
Summarized, my understanding of Virginia law indicates that a defendant commits capital murder when a death results in connection with another felony - say, robbery.
That's what Powell was convicted of in Round 1 - death in connection with robbery and rape of the younger sister. It's that conviction that was reversed by the Virginia Supreme Court; I have no idea why because I haven't read the opinion. The Va. SC also found that there existed no cause to retry Powell.
Then Powell wrote the letter.
The State subsequently recharged him with capital murder. But, this time, the state based the charge not on robbery, but on attempted rape of the murdered victim.
Double jeopardy doesn't bar a second trial on the same transaction, if the factual basis is different. In Round 1, the state had to prove that the murder occurred in connection with a robbery and rape of the younger sister; in Round 2, the state had to prove that murder occurred in connection with an attempted rape of the murdered victim.
Totally different facts.
The defendant must have watched that movie with Ashley Judd that didn't really do justice to the clause of the Constitution for which it was named.
Read the first few paragraphs of the following link (the 4th Circuit's opinion, for which the USSC has denied cert.), and decide for yourselves whether this defendant forfeit his right to live.
http://pacer.ca4.uscourts.gov/opinion.pdf/083.P.pdf