Clubs, rocks, and hands have killed more people than all other weapons combined I suspect. A friend of mine killed a man with a .444 Marlin rifle. A 444 will definitely kill a man but this one was killed when he was hit over the head. The report called the skull "shattered".
Baseball bats are considered deadly weapons in some states. If you have one in your vehicle or by your door you had better have a glove and ball with it. Good reason, they kill quite easily. There are as many soft areas of the skull as hard or more I would guess. Can kill someone with a blow to the face, the temple, the top of the head immediately behind the hard part in the front, or the back of the skull. A full power shot with a bat or cue could kill them if it connects pretty much anywhere on the skull at an angle most of the force isn't lost to glancing off and continuing the swing.
Happens fairly regulary, somebody hits somebody with a baseball bat, cue stick, iron pipe, golf club, or any of countless things we see people beat on each other with in the movies and on tv all the time with no major effect then they are surprised when they kill someone. I lost a cousin a couple months ago, he tripped and fell on level ground. Bumped his head on concrete I believe, not sure.
Another issue, self defense isn't a lock even if the other person attacks. A fellow racer did eight years after he was punched in a bar. He carried his defense too far according to the prosecutor and judge. Best I recall he didn't even kill the guy, just cobbled him up for life.
When you have to defend yourself make sure that the attack justifies great bodily harm or death. Never know when it will happen. Some people are softer headed than others and even a body shot can kill. People have died from blows to the solar plexis, heart, liver, spleen, kidneys . . . I don't know what all besides the obvious tender spots. Liability stays with you for life too. Somebody just got charged with murder for an encounter from decades ago. Too much liability to tote if it can be avoided.
If you do have to kill somebody in self defense, neatness counts. My friend with the 444 found that out the hard way.
Hu