What is the test for "express malice" murder?

Our review is framed by these principles: Murder is "the unlawful killing of a human being . . . with malice aforethought." (Pen. Code, 187.) " 'Express malice is an intent to kill. . . . Malice is implied when a person willfully does an act, the natural and probable consequences of which are dangerous to human life, and the person knowingly acts with conscious disregard for the danger to life that the act poses.' [Citation.] A killing with express malice formed willfully, deliberately, and with premeditation constitutes first degree murder. [Citation.] 'Second degree murder is the unlawful killing of a human being with malice aforethought but without the additional elements, such as willfulness, premeditation, and deliberation, that would support a conviction of first degree murder.' " (People v. Beltran (2013) 56 Cal.4th 935, 941-942.)