Steam: sandbagcone
1: Give person copious amounts of anesthetic, like alcohol, and then deftly surgically sever the spinal column. After all, want to minimize suffering, but can let them up and wander around.
2: Wander off into the sunset and try to go as far as possible. Wouldn't mind seeing what the other side is like, but wouldn't want to betray allies.
3: Take the more mobile ones with me as distraction fodder down the road. Old and slow people can be useful tools.
4: If they're your traditional Romero zombies, I'd go with either a shotgun, or a long pole. Buckshot is relatively common, and if they're the slow stupid zombies, you can easily just poke them down to the ground and they'll spend the next 15 minutes trying to figure out how to get up.
If they're fast zombies, I'd go with a hand gun. To my own head.