You can’t possibly know about their motivations, the character of the pagans or how the knights were viewed by the public in the conquered territories. Maybe they were viewed as quite OK compared to the people who reigned over this region before they came.
I doubt the people back than had the Polish national conscience the author of this book (and I suspect there are many others using this old happenings for their modern day aims) attributes to them.
Actually, it’s easy. TK were there to force pagans to get baptised. After last pagan country got baptised, their job was done in that part of the world, however they stayed there and acted more as an imperial state rather then humble servants of God.