Yeah, motive. Whittington was just about to go to the grand jury to get an indictment against Halliburton for...well, for being Halliburton. Because we all know Halliburton's involved somehow.
So Whittington had to go...and Karl Rove decided the best way to do this was to have Cheney take him hunting, get drunk, and shoot him in the head, thereby maintaining "plausible deniability" allowing them to insist it really was an accident...while, in the meantime, delaying EVERYTHING in an attempt to cover up the fact that the VP was drunk as planned. All so the guy they DIDN'T kill could NOT die of a minor heart attack a couple days later.
Personally, I find it more likely that Bigfoot did it. The only reason this is even REMOTELY plausible is because it's no more convoluted and stupid than the pre-war Iraq justifications.