What is the test for invalidating a death sentence for a convicted murderer?

Defendant asks us to invalidate his death sentence on the ground it was not proportional to his moral culpability. He points to his deprived childhood, his lack of a criminal record, and his military service during the Vietnam War. He also stresses the weakness of the evidence that the murders were premeditated, noting that in the appeal from his first trial we described the evidence of premeditation as "far from overwhelming," although we found it sufficient to support defendant's murder convictions. (People v. Lucero, supra, 44 Cal.3d at p. 1020, 245 Cal.Rptr. 185, 750 P.2d 1342.)