Exo 20:13 is dealing with murder, not a justified killing. There are times when the Lord ordains killing: wars, punishment for crimes, etc.

Now, you asked why would God kill someone who was hell-bound and not someone who was heaven-bound? After all, once someone is saved, it makes sense that they would want to leave this perverse and wicked world to live eternally in heaven, right? Wrong. The saved have a mission in the world. We are to take the gospel of Christ to the world so that others may be saved. As for those who are hell-bound? They have rejected God. Once you reject God, every breath you take on this side of eternity is a gift of God and an example of His grace. Nevertheless, there is a point when the grace of God comes to an end.