I love questions like these. The real answer is "it depends". What are you hunting, what caliber, and type of bullet are you using, have you practiced at that range, is the animal standing in a position that gives the greatest chance of success (such as broad side).
If you are hunting your game of choice with an appropriate caliber and bullet construction. Have the necessary practice to take that shot, and the animal is standing broadside then I say NO 500yds is not unethical or unreasonable. Regardless of shooting platform, field conditions, and range. It comes down to the shooters ability, and the caliber being used.
If you told me you was taking that 500yd shot with a 243 win on Elk, then yea your definitely in the wrong. But if you are using say 6.5, 7, or 300 PRC, 270 WIN/WSM, 6.8 Western, 7 rem mag, 300 win mag, etc, then I would say your are fine as long as you have the real world practice of shooting that far. Its one thing to sit down run numbers and true it up, its another to sit at the range day in and day out taking shots at those distances on a regular basis to build up the confidence of being able to control nerves, feelings, and whatever else comes up to take that shot in the field.
I've seen it personally with friends and family. They sight in the rifle, take some shots at range to get drop data and then never touch the rifle till hunting season. They have a nice 10pt walk out at 150-200yds emotions hit them and they whiff the shot. They can hit it at the range when they shoot the once or twice a year but they don't do it enough to be consistent with it. To able to control themselves so when things spike they have the repetition to take the shot.