Did you read the link posted about why it was put down though? It had been "hazed" numerous times in the past (shot with paint ball guns, and bean bag rounds) due to being too aggressive with park visitors. They also said the Elk had already been relocated to another area of the park, but it made it's way back to the same area again. Humans feeding the animals, and having no fear of humans, makes them become more brave than normal. If dumb asses would obey some simple park rules (like not feeding wild animals) maybe they wouldn't have some Elk becoming aggressive. What if that Elk had done that to a little kid? It's a risk the park can't take.
I agree with the latter part of your post about. However, I saw no signs of real aggression in the original video. Just some simple playing. An aggressive elk will kill you, spike or not. If they feel threatened, they won't run that easily.
But I do agree, people complain why animals have to be out down in a nature park, while still continuing to feed them like pets.