According to popular atheist writers, religion is a Very Bad Thing. It causes divisions, cruelty, and wars. Richard Dawkins writes in his best-seller, The God Delusion: 'Imagine, with John Lennon, a world with no religion. Imagine no suicide bombers, no 9/11, no 7/7, no Crusades'.
Note how the Crusades are lumped with terrorist slaughter.
Later Dawkins writes: 'In this book, I have deliberately refrained from detailing the horrors of the Crusades'. Christopher Hitchens in his God is not Great links 'the jihadist assault' with 'the blood-stained spectre of the Crusaders.'
My guess is that these authors reflect popular opinion.
Were the Crusades that bad? First we need to clarify which military operations were Crusades. Today the word 'crusade' is often used to refer to any program undertaken vigorously. Originally it referred to military operations undertaken by Catholic Christians at the command of a pope or with his approval. Men who were signed with the cross (Latin crux), thecrucesignatos, were called 'crusaders'.
There were many Crusades - against pagan tribes in Northern Europe, against Jews, Mongols, and heretical Christians, to speak of a few. It is quite difficult to define a Crusade because these wars had varied objectives, occurred in different centuries, and under widely varied circumstances.
However, the most famous are those undertaken to conquer the Holy Land (Palestine) in the period 1095 to 1244, so I shall restrict consideration to these.