The UN isn't that bad. It has an admirable mission and I think that, generally speaking, they behave in a decent manner. However their actual powers are very limited.
People often seem to blame the UN for things that aren't their fault, for example, the IMF is technically part of the UN but the UN really has no control over it and they act independently. The parts that the UN does control directly are usually fairly good.
That said, obviously they fucked up in Rwanda but that wasn't really 'the UN's' fault, it was the west's fault for vetoing any involvement.