Ideally yes. But the world is not ideal
But if you have a despot like Saddam violating international law the way he was how do you stop him from doing so? Breaking international law to remove someone else breaking international law that's a bit of a dilemma, Putting theory over practise don't always work
You may be happy with the US being the world police but you have to understand that the vast majority of people in the world are not. In fact the majority of the world see the US as the biggest threat to world peace.
The UN while far from ideal and in need of much reform is still the only route to authorise military action. Most of the problem with the UN is that the US often bend it to their will by economic bullying of smaller countries, or use their veto to prevent any action against allies of theirs. The US veto is wielded disproportionately especially where Israel is concerned.
The US government cannot decide who is or isn't a despot who needs removing and surely if you thought it through you would realise that. Their alliance with Saudi Arabia makes anything they have to say about human rights abuses and lack of democratic freedoms elsewhere redundant. You cannot take seriously anyone who can ally with the Saudis and claim simultaneously to support democracy and human rights. It is truly beyond satire.
Decisions about serious international affairs that affect the world cannot be left to whoever a half informed brainwashed US electorate elect as their President and surely this point shouldn't need debating. It is self evident that it is intolerable for one countries leaders to bend the world to their own will.
If the UN and international law are insufficient to solve the world's problems then reform them but starting wars and invading countries only ever makes things worse.