Has the role played by the United States in the Middle East since World War II been largely positive or negative?
There are arguments to be made on both sides of this issue.
On the negative side, the US has done things like helping to prop up reactionary and oppressive governments in the area. As an example of this, the US has helped keep the Saudi monarchy in power. It also put the Shah of Iran in power, leading to his oppressive reign. By doing this, the US has made it harder for people in the region to enjoy democracy and human rights.
On the other hand, the US has at times been a force for good. Perhaps the clearest example of this came in 1991 when the US drove Iraq out of Kuwait after the Iraqi invasion of the previous year. It is also possible to argue that the US presence in the area keeps it stable and prevents things that might otherwise happen, such as a war between the Shi'a and Persian Iran and the Sunni and Arab Saudi Arabia.