I am no expert but i know i would rather hit low instead of high on a quartering away shot, from what you explained, the deer might have just developed a blood clot after about the 100 yrd mark with the arrow still in him. If you hit the offside shoulder on a quartering shot it should be a dead deer. I wouldnt feel too bad just yet. Make sure to go through the whole area with a fine tooth comb and you will be fine. good luck.