What about the millions of innocent people the U.S. has killed in the name of "Democracy." One example would be the backing of the Guatemalan Coup in the 70s-80s that resulted in the death of 100k indiginous people in that area. These people just wanted their land back to grow some food. However does this act (and many many more) discredit all the positive things in the U.S.
Of course not, just like Castro killing his some of his opponents doesnt discredit what he has done. Have you ever read "The People History of the U.S." by Howard Zinn? If you are truly interested in the history of the U.S. then I suggest you check it out. What we are taught in schools, and hear on television is just one side of the multi-faceted truth.