The UN is a diplomatic organisation, it helps countries talk to each other. The point of the UN is to remain NEUTRAL, no matter what. Because of this the UN can stay an observe countries that have very hostile governments.
The UN is NOT a country, it does not have its own army, rather other nations fight by its name. You cannot expect it to go into countries and solve every problem through military means. There is the UNSC, but this relies on all members agree to intervene, which is rarely the case as member usual veto decisions based on national interests.
Since the UN is a diplomatic organisation and is made up of many nations, by accusing it of genocide you accuse the whole international community of genocide. Off course whether that is true is up to u.