In typical fashion, he was a not-so-holy man leading a life that was destining him to Hell. While out hunting one day, he came across a magnificent stag and decided to pursue. When it turned to face him, he noticed a crucifix floating above it's head between it's antlers and then he heard the voice of God tell him he needed to change his ways and to lay off the shrooms. He did this and apparently decided to not be a sociopathic hunter anymore by declaring people would kill their prey humanely, with the cleanest shot possible so that the animal did not suffer. St. Eustace did the same exact thing even earlier, apparently, proving beyond a shadow of a doubt that the Jesus Stag is real, but very little record of it remains, because they were all mostly pagans back then and set on fire.
He became the patron saint of hunters, mathematicians, and thirty other unrelated occupations (like most saints). He was also called upon to cure rabies until they realized priests kept getting bit by infected people. What is most remarkable is that this magical Christ-bearing stag appears on every bottle of Jagermeister.
It's a sin not to have a shot,
unless you're a mathematician.