The cause (IMO) is a human who lacks self-control, anger-management, moral compass, or is otherwise broken in a way that causes/allows them to intentionally take the life of a fellow human.

The gun/trigger/bullet/gunpowder/whatever is a mechanism of the murder, not the cause. (I'm not a rabid "gun nut" [nor even a gun owner], but blaming guns as the cause is just as fundamentally misplaced as blaming a baseball bat, knife, or rope, IMO.)