Even after knowing that Kurt Waldhiem was a top nazi official, he continued serving as United Nations Secretary General. Sometimes, people like to befriend evil hearted tyrants.
Why not we use the same yardstick for Hitler? He was not an ordinary man. From an ordinary street roamer to the worlds' most dreaded tyrant, he used all his skills to uplift his country, Germany. From rubbles to the worlds' strongest country, he conquered chaos and armed his country in just six years. He restored the German pride from the shameful (for Germany) and infamous Versailles Treaty. With that, he was able to fight for six more years... really astonishing. He was the second ruler to send his army into Russia. Had providence not intervened, he would have built a large German empire. He was a good motivator till his death. Good or bad, the achievements of a man should be praised. His child/ boyhood experiences would have driven him to become a tryrant.
But it is a fact that the present worlds' political setup is Hitler's legacy. UNO, decolonisation, cold war, America's super power status, etc.