Can you kill someone if you shoot a gun in the air?

Yes.

It's actually quite hard to fire absolutely straight up, the bullet will usually take off at a certain angle, a trajectory. And with a bit of bad luck, that bullet may end up killing someone as far as three miles(depending on the caliber) away from where it's fired.

It's good practise never ever to fire a gun unless you know exactly where the bullet is going to end up.