If a foreign leader has the power to create political unrest to such an extent that world peace is threatened and there is a risk of starting a war that endangers the lives of millions of citizens around the world, it is legitimate for government to support that person's assassination.
Governments should support the assassinations of foreign leaders if such is necessary for the protection of their own people, and the betterment of humanity. At times other governments may become hostile and threaten peace, and I believe it is a government's first duty to keep it's people safe though any means necessary. The assassination of a leader could spare the lives of many more, and to me is morally sound f the foreign leader isn't.
Planning to assassinate a leader of another society, whether it be a country, metropolis, or a humble village, is risky business, to say the least. If you're going to do this, NO ONE has to know outside a small group of people that are within government. Governments should not share plans for this, but they should be supported in only the most extreme of cases.
In the case of Hitler and of other dictators like him, having an assignation plan can help bring about peace and freedom for other people. There is no reason why a governtment should not support assassination in such extreme cases as this. Sometimes it can safe lives and end wars.