Revealing mistake: Right after a fleeing felon is shot dead and falls into the pool (whereupon Murphy says "He's bought it"), he appears to be swimming to stay afloat. (00:12:10)
Revealing mistake: When the van gets flipped over outside the BBQ chicken place, you can see the cannon that flips the van over. (01:32:20)
Chosen answer: Murphy has been deemed a threat to the public at large by the authorities, having "snapped" and stolen an armed helicopter. Cochrane is using that determination as cover to finally kill Murphy, whom he's long despised. Killing Murphy on the ground would be harder to get away with. He would not be responsible for paying for the helicopter anymore than the Air Force or the other police helicopter crews would be had they sucessfully knocked Blue Thunder down.
johnrosa