Play nice gentlemen. Good shot for the OP. Some might complain it's a little far back but the same thing could have happened with a much closer shot too. The OP shooter could have tried to get closer with that ridge to his left but the buck could have busted him and/or just happened to get up and move anyway. That's why they call it hunting.

Everybody has their own opinion about what is an ethical range to shoot animals and what's not. Everybody has their own particular limitations as a rifle shooter too. Some people are challenged at even 100 yards and the OP is apparently not at 670 yards. Just because it's not your cup of tea whether through ability or just personal beliefs/ethics, etc. doesn't mean you should attack another hunter for doing something successfully. I happen to remember when Zumbo wrote an article bashing AR-15's, saying they had no hunting use or application. It was his opinion and he was certainly entitled to it but it was also unwise and didn't reflect well on hunting (erroneously as it turns out).

I don't know that I'll ever shoot an animal at 670 yards (400 yards is my longest shot to date) but I don't necessarily know that I won't either. If the OP was wounding a bunch of animals, missing them, etc. at that range, I'd say he was in over his head and should seriously reconsider shooting at that distance. Since he hit the animal, killed it and recovered it pretty darn close to where he shot it, I'd say he's capable at that distance and it's pretty much a personal decision on his part to shoot at that range. I'm definitely not going to bash him for it and instead offer my congratulations on a good shoot and a very nice muley.