Right, I'm an engineering student, so I'm going to create a metaphor using imagery which I am comfortable with, just bare with me.
Lets say that I design a large high altitude passenger plane. I convince someone to pay for the construction, and even have it to the point where it can be flown. Soon the big day arrives, and hundreds of people fill the seats as my plane makes it's first ever trip. However, I forgot one essential thing, an air supply. The pilot blacks out in the thinning air, and the plane crashes.
Now, the plane worked perfectly, there was no problem with my plane. Only one thing went wrong, and that was because of people, not my plane. Yet the entire plane is now charred wreckage.
Yes, the United Nations is bigger than just the security council, for example, we have the UN Human Rights Commission on topic here (you might even be accused of missing the topic itself). Yet, we have a major fault in this Commission, the fact that it is made up of countries which fail at Human Rights themselves. For example, the United States is a country in full support of capital punishment (I am as well), yet many countries don't agree with this stance, why is the United States on this Commission if it violates such rights? Then another step down to genocide in Sudan.
To hark back to my rather elaborate (or at least long) metaphor, the Security Council and the UN's functions as peacekeepers, is a major component of the UN. Yet, this component is one which regularly fails, and could quite well cause the collapse of the UN (in another World War, or if the Cold War ever erupted).