Saddam was as ruthless as tyrants come. He tried to ethnically cleanse the Kurds and other minorities in the country with mustard gas and mass executions. We are still finding mass graves to this day. He imprisoned any political opponent and tortured them. When we invaded we found prisons with tens of thousands of starving, beaten, and scared people whose only crime was opposing Saddam's policy. He was the largest sponsor of terrorism in the world. He suppressed the Sunni majority for the benefit of his own Shia minority. He interfered repeatedly with oil trade in the Persian gulf, threatening the economies of every nation that was involved with trade there. His only benefits to us were that he kept Iran in check and he helped us with anything we needed in the middle east (we did put him in power after all). But once we established better connections with Saudi Arabia, Saddam's Iraq out lived his use.
George Bush Sr. Could have supplanted Saddam Hussein during the Gulf War, but he didn't. He was aware that overthrowing him would have destabilized Iraq and forced us to occupy the country. So what makes Bush Jr. Justified? He only invaded Iraq because he felt as though his father made a mistake in not dethroning Saddam, and he thought he was undoing that. When it comes to making decisions like this, it is imperative that we examine the costs in addition to the anticipated benefits.