He likely used the claw side of a claw hammer. This would likely cause distinct double, beveled rectangular puncture wounds extending into the calvarium, puncturing the meninges and extending into the brain. Leaving a distinct, wide wound channel like a puncture, but also like a chopping would, but in an interrupted "dotted or dashed line" form, two dashes at a time. A true sharp force trauma injury that is a combination of a puncture and chopping injury.
Other speciality hammers (Like a Welder's chipping hammer or a geologist' s chipping hammer, both of which have a blade like end with no gap in the middle), would likely create a more classic chopping wound, but with a possible deep penetrating aspect like a wide broad puncture wound, depending on the force used.
The hammer side of a hammer would cause a flat round, blunt force possibly depressed injury to the skull and meninges. A rounded ball peen hammer will create a more rounded, pit like fracture, possibly driving bone shards into and through the meninges and into the substance of the brain. I feel that the ball peen hammer is the more deadly of the two.
JMMO
