King David's blood

 John MacArthur was citing all the sins of David, in an effort to show that God is merciful, and listed among them the fact that David had "slaughtered so many people that God wouldn't let him build the temple", which I don't see in The Bible.  God prohibited David from building the temple because God was trying to make Himself known as a God of peace, vs man who is constantly at war with himself and those around him and needed (and still needs) to figure out HOW to seek peace, and then go about doing so.
War is a necessity, but it is not the ideal.  God IS THE IDEAL.  He was wanting to reveal Himself to the world, not just Israel.  The world around Israel had heard about God, but by that point people were probably terrified of Him, thinking He was just some bloodthirsty hate monster.  God didn't want the vessel with which that fact came to be so closely associated with His house.  He wanted His house to be a much more inviting place, a place where people weren't so burdened and terrified to approach.

At least that's what I gather from the text.  I don't see any indication that David committed any war crimes.  God didn't send Nathan to scold him about any such thing.  He did send Nathan to scold him about killing one man, because he was killed for no reason except so that David could conceal his other sin(s).

JM also cites "polygamy" as another of David's sins...polygamy was not any more of a sin in Old Testament times than eating pork is today.  Jesus insisted that people should have only one wife, so that should be adhered to.  But God didn't make any command regarding qty and marriage and in fact, He made provisions with how to handle it.

I agree, we should not minimize sin or encourage others to do so.  But we don't need to label any and every possible thing as being "sin".  If God wanted David or any of the other Israelites to marry just one person, then God would have told them "Only have one wife" (or something to that effect).  God didn't do that.  Sin is not the same thing as God being displeased with someone's actions.  Sin is violating God's command.  The early chapters of Genesis do not give COMMAND regarding marriage.  It only says that Adam & Eve had sex and bore children.  People talk about "God's design", but in that case, what about computers?  I'm typing this thing on a computer that uses electricity.  That is NOT within God's design.  I seriously do not believe that Jesus came down from Heaven to sacrifice Himself for humanity so that I (& others) can be forgiven for lending approval to the paved roads that I and many fellow travelers use, to get around to different places.