What punishment is appropriate for a life-sentenced killer who kills again?

According to Arkansas News Online, Kenneth Dewayne Williams was sentenced to

life without parole for the Dec. 13, 1998, kidnapping and slaying of 19-year-old Dominique Hurd, a University of Arkansas at Pine Bluff cheerleader from Fort Worth. Williams was convicted of capital murder in Hurd's death and of attempted capital murder in an attack on Hurd's boyfriend, Peter Robertson of New Jersey.

According to trial testimony, Williams abducted the couple from a Bonanza restaurant parking lot in Pine Bluff, forced them to drive to an automated teller machine to get cash and then took them to a wooded area and shot them.

And what happened next?  On Oct. 3, 1999, 18 days after beginning his prison sentence,

Williams ... escaped by hiding in a hog-slop truck that was on the prison grounds and then jumping out as the truck left the prison. He hid in a ditch and eventually made his way to [Cecil] Boren's house, where Boren was outside working in his garden. Williams stole Boren's guns, fatally shot the former warden seven times and made off with the guns and Boren's vehicle.

Williams was recaptured the next day after a high-speed chase in Missouri that resulted in the death of another driver.

Williams really should have gotten the death penalty the first time.  Kidnapping, robbery, and attempted murder on top of murder are more than sufficient aggravating circumstances.  For the second murder, the choice is even more clear.  A life-sentenced inmate is "judgment proof" from any sentence but death, as life sentences are necessarily concurrent in fact regardless of how they are sentenced on paper.  The choices of punishment for the murder of Cecil Boren are (1) death, or (2) none at all.

Option (1) is scheduled for overdue completion tonight.