"I thought I was an alright bow hunter, but maybe I've just been lucky in the past. I think I made a really dumb shot this past weekend."

Two things strike me. One, you saying maybe you have been lucky in the past? How so? On low percentage quartering to shots? And two, your mind must be playing tricks on you if you saw an arrow enter near his back, but in reality it hit low in the shoulder, front side?

I hope you learned a good lesson here.