Apparently, it is improper for prosecutors to argue to jurors that the only way to be sure the defendant doesn't kill again is to impose the death penalty:

http://www.rgj.com/article/20081206/NEWS/812060345

JAS

The prosecutor didn't just say the defendant could kill again. (Which includes killing in prison, escaping, etc.) He outright said the defendant WOULD be released despite a "life without parole" sentence and the LWOP system was a "fraud". I don't really see many courts accepting that, whether it's the 9th or the 5th.

The case is from 1983. We needed 25 years of litigation to discover this issue? Doesn't sound like defense even objected to the argument. (By the way, is it ironic that the prosecutor's argument resulted in that which the prosecutor warned could happen?)