"Just because it is hard doesn't mean it shouldn't be done."

It may also just be impossible.

"If all the effort to take guns away from people were placed on trying to change human nature"

Religion and governments have been trying to do just that throughout all known history and they have used weapons to try doing so. How well is that working out?

"then maybe we wouldn't need guns any longer."

I don't need a gun.

"Maybe someone should invent the replicator."

Maybe someone will. If all the effort that goes into making weaponry was placed on technological advancements for the sake of humanity perhaps it would have been by now.