The authors of the Constitution did have a good reason for armed citizenry. It was to ensure that the citizens could protect themselves from a tyrannical government, and even rebel against that government if necessary.
Historical background:
1. US: During the American Revolution from England, the colonies' rebels had very few guns. They used pikes and pitchforks, or whatever they could find. Most soldiers brought their own personal guns with them. There weren't enough guns for the military to issue them. But the volunteers who did have their own guns used those, and also the experience they had in handling the guns. The other unarmed volunteers, when they finally got guns, needed a lot of training because they were inexperienced with guns. Our Constitution's authors didn't want to let that happen again. They wanted the people themselves to be ready to take up arms against tyrannical leaders.
2. World: The German government ensured an easy collection of Jews and dissidents by collecting up all their guns first.
Unarmed civilians are not just at the mercy of criminals but sometimes at the mercy of their own governments.
Hunting, target shooting, home defense, and gun collecting aren't the reasons for our 2nd Amendment but they support the readiness of citizens.
Americans are also free to not have guns. No one is forced to buy or use a gun. For those who don't want guns in their homes, I respect their decision.