Perhaps they, too, were a conquered people at one time, rising up against their oppressors and taking a more outward aggressive militaristic stance in the aftermath.
Trauma often is a cycle. But at the same time, we don't know that anyone ever did conquer the Cardassians. The impression I always had was that Cardassia ran out of natural resources and Cardassian society fell to pieces until the military took control and focused their entire civilization on taking resources from wherever they could find them.
This. Everything on screen indicates it was the depletion of resources which pushed Cardassia into becoming Space Nazis.