International law existed long before the UN, and the UN was never intended to be a world government with enforcement powers.
The UN is a forum, where problems are identified, solutions are proposed, and actions taken voluntarily. International law involves treaties and agreements between nations that are separate from the UN, although their paths cross often.
The first Geneva Convention, for instance, was in 1864-- well before the founding of the UN.
http://www.genevaconventions.org/The failure in Rwanda was not the UN's per se. It was a failure of international commitment to stop the killing. Within the UN, there was massive outrage at the genocide, but many of the individual members preferred to do nothing and leave it to Africans to deal with African problems. As Africans dealt with Idi Amin and others.
The present mess in the Congo, the slave trade, child labor, and the illicit diamond trade are other examples of the same talk in the UN and attempts by its relief agancies to help, but failure of major powers to take action.
The old days of UN blue helmets with Irish, Kenyan, and other troops from nonaligned nations rushing in seem to be waning.
All countries rotate on the non-veto Security Council seats and other committees, and our impression of them has little to do with whether or not they should be there. Kind of like a Californian weighing in on how Tom deLay's district should vote. Many of those assignments and votes were a deliberate slap at the US which insisted on trying to run things while not even paying its bills. Such games might not be nice, but should be expected.