Second-degree murder (ARS §13-1104) is ordinarily defined as:

1) an intentional killing that is not premeditated or planned, nor committed in a reasonable "heat of passion" or

2) a killing caused by dangerous conduct and the offender's obvious lack of concern for human life.

Second-degree murder may best be viewed as the middle ground between first-degree murder (which is planned & intentional - ARS §13-1105) and manslaughter (killing someone unintentionally ARS §13-1103).