There is nothing wrong with your setup or the broadhead. Your KE should be 55# or a little better which is plenty to do the job. Shot placement is everything. Hit a little high-low-back-forward and you can hit bone. You might have even hit bone on the backside depending your shot angle.

Bottom line: Your BH shot true, and created a fatal wound with an easy recovery.......the ST did it's job. Don't sweat the details.