What factors preclude the formation of malice and reduce murder to voluntary manslaughter?

"Two factors may preclude the formation of malice and reduce murder to voluntary manslaughter: heat of passion and unreasonable self-defense." (People v. Elmore (2014) 59 Cal.4th 121, 133.) "Self-defense, when based on a reasonable belief that killing is necessary to avert an imminent threat of death or great bodily injury, is a complete justification, and such a killing is not a crime. [Citations.] A killing committed when that belief is unreasonable is not justifiable. Nevertheless, 'one who holds an honest but unreasonable belief in the necessity to defend against imminent peril to life or great bodily injury does not harbor malice and commits no greater offense than manslaughter.'" (Id. at pp. 133-134.)