Well, you can thank John Sheridan, Delenn and their so called Alliance for that. While there should be some truth to the ruthlessness of the Shadows, as is well documented by the Royal Library of Centauri Prime (the revised edition of the History of second Molari dynasty, published after the end of the planet's isolationist period), there is no question that the Alliance have produced lots of confusion through sheer propaganda, but more than anything because they have never had a transparent, democratic process. Much of the contemporary sources on the Shadows have been distorted by being almost exclusively oral in nature; the command staff of Babylon 5 in the 2260's (later being renamed the Alliance) never kept any official records.

Source: https://www.youtube.com/watch?v=DrPe80sxlm4

That said, there is ample evidence that the Shadows, together, or at least in tandem with, the Vorlons commited great genocides and planet destruction before they suddenly dissapeared from the galaxy. The difference between a Volron incident and a Shadow incident is that the Vorlons completely destroyed the core of the planets while the Shadows only attacked the surface.