Yes, the US invasion of Iraq was justified, because there were significant risks. The intelligence that the United States had at the time showed significant risks for weapons of mass destruction. In addition, the regime in Iraq at the time was very oppressive. The United States was correct in wanting to mitigate these risks before they caused additional harm.
Those who voted for the Iraq War were either duped by W's warmongers or just stupid. Saddam Hussein was incapable of inflicting damage to the United States with any of his hardware. The missiles he had could only reach as far as Israel and whatever chemical stockpiles Hussein had wouldn't be able to reach American soil. To claim Hussein was responsible for 9/11 in some way was ridiculous--by 2003, everyone already knew Osama bin Laden was public enemy number one.