Humans have no doubt done several HORRIBLE things, but I think that the worst thing people have done in human history is kill each other and put each other down. This is pretty clear in World War II when thousands of people (especially Jews) were discriminated against, put to live in inhumane conditions, and butchered mercilessly.
Another example of this is during the periods of American slavery, when African Americans were treated like property and suffered great inequalities! That was horrendous!