The worst atrocity is not following the teachings of Jesus Christ and then claiming to be doing one's own selfish and hateful actions in HIS name.
Suppose someone went to your spouse or best friend, assaulted them and then claimed to be doing it at YOUR command, because they were YOUR follower. More people do the same. And then people start asking "What is the worst atrocity associated with believing in YOU?
Jesus said, "'Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment. And the second is like it: 'Love your neighbor as yourself.' (See Matthew 22:37-39) He also said, "So in everything, do to others what you would have them do to you." (See Matthew 7:12)
And this is what he said to those who claimed so falsely to be religious and self-righteous: (See Matthew 23)
"Woe to you, you hypocrites! You shut the kingdom of heaven in men's faces. You yourselves do not enter, nor will you let those enter who are trying to. You travel over land and sea to win a single convert, and when he becomes one, you make him twice as much a son of hell as you are."
The problem isn't Jesus Christ, or TRUE Christianity. There you will find a higher standard of love and compassion for others than most of us will ever reach.
The atrocity is the selfish failings of people who fall so far short of the simple commands Jesus gave us. People do all kinds of ugly, selfish, hateful and self-righteous things, in the name of their religion, their race, their government, their political party, and even their favorite sports team, or just to feel good about themselves. None of this has anything to do with Christ or being his follower.
It's always easy to find someone in a group other than your own, criticize them and be blind to one's own self-righteousness in the process. We could all do better to learn a bit more about what Jesus actually taught and to then follow it.