Here is an argument made by Geoffrey Robertson in support of a right, indeed duty, of humanitarian intervention in certain circumstances, even without the backing of the UN Security Council:
There are these situations where you can legitimately bypass the Security Council, which is blocked on political grounds, and act as the UK, United States, and France acted without Security Council authorization to set up no-fly zones in Iraq to protect the Kurds in the early 1990s. The bombing of Kosovo in the early 1990s to stop ethnic cleansing was another example of acting without Security Council support. There are these narrowly defined situations where if you act proportionately, if you have a prospect of success, if you are acting to stop crimes against humanity at the behest of the people who are being killed, then I think once those preconditions are satisfied state alliances and states are justified, and indeed intervention becomes lawful.
A question that might arise here is this: if the rules of the international system specify that authorization by the Security Council is required for humanitarian intervention, how can 'state alliances and states' legitimately bypass it? Doesn't this mean they're taking the law into their own hands? The answer to the latter question is yes. But it's no different from the situation under domestic law when a state either acts in gross violation of the rights of those under its jurisdiction or fails to protect such rights. No one is bound by any system of law come what may. In democratic societies we accept a certain amount of what we regard as wrong because it is the presumption of democracy that, within certain moral limits, majorities are to be allowed to prevail. But beyond those limits there are rights of civil disobedience, rights of resistance and, in extremis, a right of revolution. Within the international system, equally, it is morally intolerable that, say, people may be freely massacred by their own government just bacause a veto-wielding country decides for political reasons to exercise its veto in the Security Council.
The big question is: how do you know you're justified in moving to uphold norms that trump the powers that be - whether 'you' are a citizen acting in defiance of government, or are a state or alliance of states acting without Security Council authorization? The answer is that you don't always know for certain. That's why there's so much dispute about these matters. Guided by the normative principles that apply to them, you make the best stab at it you can - or, in John Locke's terms, you make appeal to Heaven.