The deformation of the aluminum bat leaves it in contact with the ball longer, and the spring action transfers more energy. A human head will deform more easily

The deformation of the aluminum bat leaves it in contact with the ball longer, and the spring action transfers more energy. A human head will deform more easily than a baseball; blunt impact injuries are a function of momentum over time, so a bat that deforms on impact would do less damage. Any baseball bat can be used to kill by hitting people in the head though.