By Michelle Nichols, Humeyra Pamuk and Doina Chiacu UNITED NATIONS (Reuters) -Ukrainian President Volodymyr Zelenskiy told the United Nations Security Council that “accountability must be inevitable” for Russia as he accused invading Russian troops of committing “the most terrible war crimes” since World War Two. Zelenskiy showed a short video of burned, bloodied and mutilated bodies, including children, in Irpin, Dymerka, Mariupol and Bucha, where Ukraine accuses Russian troops of killing hundreds of civilians. Russia’s U.N. Ambassador Vassily Nebenzia then told the Security Council that Russ…