Watching The Big Lebowski again, for the Nth time, I felt a bit silly.
Pretty much everything that Dude brings Walter in on, gets screwed up. Yes... throwing either ringer would have resulted in the same thing... but that's what started his car getting wrecked... and shot... etc. In one sense, if Dude and Walter hadn't done things the way they did, they never would have found out the truth in the end. However, it was Walter's interventions/plans/decisions that ended up putting the nihilists in that parking lot with Donny, to cause the situation that brought on his heart attack.
So... did Walter kill Donny? Even inadvertently...