The worst thing we've ever done?
Eugenics
The idea that we, as a people, would try to eliminate "undesirable" aspects of the human species by killing/sterilizing the "unfortunate" members of our society cursed with these aspects is madness and dehumanizing. By the mid-20th Century this mentality had led to 60,000 forced sterilizations in the United States and The Holocaust in Europe.
I would like to toss a little nugget out there for those who think that religion is the worst thing because of all the deaths it has caused.
People will use anything as an excuse to fight people that they don't like. Religion is simply being used as a flimsy form of 'justification' for horrendous activities, but to blame religion for causing the issues in the first place is misguided.
For those who are dead-set in blaming religion, here's another nugget: The only thing that has killed more people than religion is the lack of religion. Hitler, Stalin, Mao, Pol Pot, Myanmar, Darfur, Rawanda, Sudan, Yugoslavia, and the numerous incidents of genocide that take place in Africa and the Middle East in modern times are/were perpetrated for political influence, not religious reasons. History has proven that people who are driven to seek power will drift towards immoral activities if they feel that they don't have to answer to a higher authority in the afterlife.
People will use anything as an excuse to fight people that they don't like. Religion is simply being used as a flimsy form of 'justification' for horrendous activities, but to blame religion for causing the issues in the first place is misguided.