Hi,
You can say that God has given life to us and He alone has the right to take it back again. He calls each one of us to Him when He thinks it’s the right time. Well He thought it was the right time for those people to die. Joshua only killed them because God told him to. He didn’t do it on his own because we ordinarily don’t have the right to decide when other people should die. Certainly, we have a right to protect the innocent and to defend ourselves. But I suggest not getting into that until he is ready to ask aobut it.
Fr. Vincent Serpa, O.P.