Nobody is perfect, and few organisations are irredeemably corrupt or evil. The UN is no exception.
On the one hand, the UN refugee agency has been trying to do good work on the ground in Sri Lanka in the face of a huge refugee crisis, when the government allows them. (They also do vital work in Pakistan, the Sudan / Darfur, Iraq, and elsewhere).
On the other hand the UN Human Rights Council has ducked the question of human rights abuses and possible war crimes during and after the conflict in Sri Lanka, calling it an internal matter. But as I mentioned before, considering who is on the UNHRC, and their sensitivities about their own "internal" affairs, we should probably not be too surprised.
At least parts of the UN are aware of the need for rapid resettlement of refugees, and the importance of not indiscriminately shelling concentrations of civilians.