This is the verse - KJV:
15. And Nathan departed unto his house. And the Lord struck the child that Uriah's wife bare unto David, and it was very sick.
With very many infants lost to death during their first few days of life at this time period; were they ALL "KILLED" by God?
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I gave you the chapter, but you snipped the verse and tried to confuse the issue. Let me help you, it's really not that hard with the whole context.
Nathan, God's prophet, tells David exactly why the child will die.:
2 Samuel 12: And Nathan said unto David, The Lord also hath put away thy sin; thou shalt not die.
14 Howbeit, because by this deed thou hast given great occasion to the enemies of the Lord to blaspheme, the child also that is born unto thee shall surely die.