Sometimes you have to take the good with the bad. Given the opportunity at a walking deer you have 2 choices, If you feel he will NOT stop by himself. If this is a deer from the area and you will get another crack at him, let him walk. If this is the first time he has been in the area as far as you know, better to chance the spook than hit him in a bad place from walking too fast and never finding him. Split second decisions happen all the time, usually nothing goes as planned!
Dandbuck