Trump has no responsibility to the rest of the world
A government's job is ultimately to protect its own citizens, and President Trump's attempt to purchase the COVID-19 vaccine is a reflection of such a duty. Assisting all citizens of the world should always be an aspiration, however, it may be too idealistic. Instead, it is simply far more feasible for world leaders to pursue what is in their own countries best interest. This is especially true within the United States when considering the fourteenth amendment. In other words, the fourteenth amendment states that all born in the U.S.A have not only rights to citizenship but a right to protection, life, liberty, or property. Thus, Trump's actions were in accordance with the constitution.
As an international leader, Trump certainly does have a responsibility to all citizens of the world. Nationalities are simply titles, we are all part of a bigger human collective. And due to such a reality. we should have each other's best interests in mind. It is through this understanding that peace and harmony between countries can be established. In other words, as the leader of one of the most powerful countries in the World, President Trump should promote and protect all human rights, not just human rights as they pertain to America, and not just when it is convenient. Trump should actively protect these rights throughout his presidency. What is the point of even proclaiming human rights if no world leader will follow them or call out their peers for failing to?