To be a Christian is, in a nutshell, to follow the teachings of Christ. Those who call themselves Christians but do not follow the teachings of Jesus are liars. Attributing the evil actions of these liars to true Christians or the teachings of Jesus is either ignorant or dishonest.
Jesus said that his followers will be known by their "fruit" or "produce." If you read up on what Jesus actually taught, it will become immediately apparent that the Crusaders, the Iquisitors, and the Puritans (at least the notorious Puritans) were not Christians as they claimed, for they did not follow the teachings of Jesus. Attributing the works of these false men to Jesus or his followers is, again, wrong.
No one following the teachings of Jesus made this world a worse place in doing so. And no one making this world a worse place was following the teachings of Jesus when he did it.