No regret for the animal's I take ever. That's what I'm doing there. A deer is no different to me than a bream, catfish or bass. They're all food. It's just a bigger, different version of food. My job is to harvest the deer in as quick and painless way as possible.
Never let the kid in you, fill that deer brain with emotion and complex thought. Stay focused. On your way home, one will most likely use that complex thought to jump into your radiator.

Now I have regret for animals that I know someone injured and didn't track because they were just plain lazy. Those deer that I have seen with heads removed, and the carcass dropped on the side of the road. Those that are killed, and processed, then the meat just ruins in the freezer until its thrown out. That's just a shame.