Who shot him?

OK~ let us review and back to the scene at that critical part; Hans, staggering from the shot, is not finished yet. Still holding Holly, he crashes through the window of the skyscraper and hangs on to her arm for dear life. Maclane comes to her aid and tries to prize Hans off.

All the while, Hans is slowly leveling a gun at McClane with the intention of ending his life. McClane (and the audience) focuses on the watch on Holly's wrist. Unclasping the butter hook, Hans' grip on Holly is released just at the moment he was about to shoot. At this point, gravity takes over, and Hans plummets countless storeys to his death.

After McClane tosses the body out onto the cop car, we would have to assume that the window is broken, Yes? As the cop car is being shot at a window gets shot out during the intial rounds. This is a bit confusing, but I feel based on that info that is was the bad guys shooting. As far as John being happy, its hard to say, maybe he was just amused with his line 'welcome to the party pal'. I hadnt noticed that he was amused with that before, I shall have to watch for that next time.