IMO, not an accident, but an act of pure, outright, unsafe stupidity. Right from the article...
"saw what he thought was a deer and fired his rifle"
How many times have you heard that when it comes to hunting "accidents"? I hear it multiple times per year. Unless the guy that got shot was wearing a deer costume and crawling on all 4's, there is no possible way that you could mistake a person for a deer if you are safely and correctly identifying your target before you shoot. You should not be shooting at movement, color, noise or anything other than a damn deer.
Stories like this get my blood boiling because they are called "accidents". Nobody gets shot by accident, every shooting is either intentional or a result of unsafe gun handling.