Near the end, Cochrane wants to kill Murphy. Why do it in the air? Even if Cochrane did wipe him out, the only way he would have to do that was to destroy the helicopter completely when he had the chance; instead, he injured Murphy and disabled the cannon somewhat, which is painful to watch and understandable, for fear of collatoral damage or simply because he did not want to blast it out of the sky and foot the bill. But the helicopter costs '$5 million', and even if Cochrane had the money, it would have been cheaper to take Murphy out on land instead of in public and in broad daylight. [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.]