Here's the abstract of a study on how three strikes laws encourage witness-murdering, but no word yet on any studies of whether the death penalty serves to discourage witness-murdering.
The Lethal Effects of Three-Strikes Laws Thomas B. Marvell, Carlisle E. Moody Journal of Legal Studies, Vol. 30, No. 1 (Jan., 2001) , pp. 89-106
Abstract--Three-strikes laws provide very long prison terms for certain criminals with prior convictions of serious violent crimes. It is likely that the laws increase homicides because a few criminals, fearing the enhanced penalties, murder victims and witnesses to limit resistance and identification. With a state-level multiple-time-series design, we find that the laws are associated with 10-12 percent more homicides in the short run and 23-29 percent in the long run. The impact occurs in almost all 24 states with three-strikes laws. Furthermore, there is little evidence that the laws have any compensating crime reduction impact through deterrence or incapacitation.
A reader comments:
Memorable relevant anecdote: that early scene in the film "Heat" where Waynegro (Kevin Gage) shoots dead a security guard for no discernable reason, making McCauley (Robert DeNiro) and the rest of his crew guilty of felony murder. So McCauley gives the go-ahead to Cherito (Tom Sizemore) to murder the other two guards, because there's no additional legal punishment for those two murders.
This is the flip side of the logic that persuaded the Victorians to stop hanging pickpockets -- if both the Artful Dodger and Bill Sikes are liable to be hanged, how do you discourage pickpockets like the Dodger from turning into robber-murderers like Sikes? The criminal law needs gradations of punishment to provide proper incentives.
We've discovered over the last quarter century that we need long prison terms to discourage criminality, but long terms, in the absence of a higher penalty (i.e., the death pealty) reduce the opportunity cost of witness-murdering.