Saddam was a bad guy. But you're asking the wrong question.

You asked: "I ask again, why did the UN leave this murderer in power after the first Gulf War?"

In reality, you should be asking "why did George Bush leave this murderer in power after the first GUlf War."

Keep in mind the first Bush chose not to enter into Iraq to topple Saddam from power. Why? Because he knew what kind of quagmire it would lead to.

Instead he chose to rely upon UN supervised sanctions and inspections which turned about to be very effective in containing Saddam and keeping him from threatening his neighbors and his own people. For most of the 1990s Saddam Hussein was a powerless despot, living in hiding and only able to erect statues of himself in city traffic circles. It was only a matter of time before he would be torn from power by his own people. The war was completely unnecessary.