Overall, I'd have to say negative. I'll go into more detail when I have time, I gotta run ATM though.
Going into more detail now:
I picked "overall negative" based on the way that I weigh the negatives and positives. The biggest negatives in US history outweigh all the positives in my eyes because of their "atrocity" factor. The two biggest, IMO, are the genocide of the native population and slavery.
Slavery was "rectified" in that it was made illegal, but that isn't a positive by my way of viewing things so much as it is a "return to neutral". Nothing can be done to erase the genocide. There were other things that happened in our history as well that I feel overshadow the "good". Japanese internment camps, Jim Crow, the a-bombs, the Tuskeegee incident etc.
The reason why these things overshadow the positives is that our positive history is primarily with ideas specifically in the realms of philosophy, economics, science, politics etc, while our negative things all deal with human beings being murdered, maimed, downtrodden, subjugated, etc.
In my view of the world, these negatives will always surpass those positives because human beings, individuals, matter more to me than concepts or material things.
Now, since the question that was asked refers to opinions about our past, I have to lean towards the side of negative overall because of what was affected by the positives and negatives.
Now, if the question was "How do you feel about the US's over all historical influence in the world: positive or negative" I would say that since concepts, inventions, and such have more overall positive influence globally, that I have a positive view overall of the US's historical influence on the world.
But since it was just "past" in general, I took all factors into account, including my own system of weighing positives and negatives. Obviously actions directly taken against people, be they positive or negative, have the highest weight in my personal views. We've got more negative than positive on that front by a lot. That skews my view towards the negative.