I'll recommend that you take your subject down to one crusade in particular, as the causes behind each of them over centuries are much, much different. The question of channelling violence is usually discussed principally as a reason for the first crusade, and is mostly derived from ecclesiastical sources like sermons from the period:

• Christoph T Maier, Crusade Propaganda and Ideology: Model Sermons for the Preaching of the Cross (Cambridge University Press, 2000)

If you are writing on the first crusade you cannot realistically start without familiarizing yourself with:

• Jonathan Riley-Smith, The First Crusade and Idea of Crusading (Continuum, 2009)

He's pretty much the most important living scholar in crusade studies, and this book will familiarize you with the historiographic debates on origins of / reasons for crusading and source materials used in the debates.