The Snohomish County Medical Examiner ruled the death a homicide yesterday with the cause of death being asphxiation (choking). Here is one way this could have gone down that without independent corrobaration is just as likely as the 67 year old's version.

He kills the guy (there evidently is some relationship between the two, I read that the "thief" is a friend of the homeowner's nephew)
Phones 911 and says somebody is trying to steal his outboard motor
Removes the outboard motor from his boat
"Detains" the suspect until the police arrive

Now, what makes the above scenario any less likely than what has been reported? I don't know what happened, none of us do. The only one with knowledge is the 67 year old guy. We love the idea of an older "good guy" winning in a battle with a bad guy who is stealing property. But, we need to think and not follow the herd. And we also need to remember that although getting burgled is a lousy thing, we don't categorize burglary as a Capital Offense.