The real reason is that the UN supports: genocide, slavery, and terrorism.
Thus the United States taking down a tyrant was striking a blow at one of the major pillars holding up the UN. Remember that the UN was founded in the wake of the two World Wars, and the failed "League of Nations." Making all nations safe from invasion is a big thing, but because it protects a lot of bad actors, the UN does not like to dwell on this angle publicly.
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Close the U.N. down and solve many problems. Or at least send it all to Brussels, where it really belongs.
Do everything possible to destroy the USA.
Well, Parkinson could have predicted this long since.
Actually, the primary purpose of the United Nations is to give cushy jobs to as many bureaucrats as possible. That is the primary purpose of any well-developed bureaucracy: to grow, to expand, to increase salaries and benefits of its functionaries, to have more flunkies working under you.
In order to ensure that these primary benefits continue and grow, there are other bureaucratic principles: Don't rock the boat, CYA, and share the wealth with the most corrupt members of your board of directors, who will then return the favor. That's why the Oil for Food Money was kept almost entirely in French bank accounts.