Did Saul?

Have two sons named Mephibosheth? I am not sure I understand this passage in 2 Samuel 21. Anyone have any insight?

7 The king spared Mephibosheth son of Jonathan, the son of Saul, because of the oath before the LORD between David and Jonathan son of Saul. 8 But the king took Armoni and Mephibosheth, the two sons of Aiah’s daughter Rizpah, whom she had borne to Saul, together with the five sons of Saul’s daughter Merab, [a] whom she had borne to Adriel son of Barzillai the Meholathite. 9 He handed them over to the Gibeonites, who killed and exposed them on a hill before the LORD. All seven of them fell together; they were put to death during the first days of the harvest, just as the barley harvest was beginning.

2 Samuel

2 David also defeated the Moabites. He made them lie down on the ground and measured them off with a length of cord. Every two lengths of them were put to death, and the third length was allowed to live. So the Moabites became subject to David and brought tribute.

Wow.  I just find this strange.  I don’t understand the reasoning behind killing 2/3 of the people and leaving the other 1/3.  I am  finding it interesting to read my Bible (ok, it’s Bible Gateway but still it is amazing) and to find the strangest things in it.  I am not so much amazed by the actual things but more by the fact that we (humans in 2007) have been TRAINED, BRAINWASHED, CAJOLED, etc. into believing our own little world is the “right” world.  Who do we think we are?  Why can’t we realize we can be in the world as a light and a lamp to Christ but not OF the world?  Why is this so difficult.  I know the drill, Adam and Eve ate the fruit, we are no longer in Eden…I know all of this.  What I don’t understand is how we could have come so far away from what God really wanted for us and fallen into this world full of scariness?  Don’t get me wrong, it’s pretty dogone scary to read about David killing 2/3 of the people.  In fact, what I think I might be getting at is that we no longer have the fear they had back then.  No fear.  That is scary.