See deathpenalty.org.

Life imprisonment could be equally as effective-what harm do you think the felon will do to "society's structures and individuals" from within his cell? However, the cheapening of life that results from society itself being willing to take it harms everyone in general.

If you are referring to the "deterrent effect", on the other hand, no even correlational evidence has been found of that, much less causal-anyone deterred by the threat of the death penalty would likely also be deterred by the threat of life imprisonment.

Most murders happen either in a fit of intense emotion, when the murderer is not thinking rationally at all, or by someone who has plotted not to get caught, and therefore does not see punishment as a threat.