I agree with Essex here. Legolas is shooting out of impulse, not out of calculation. After all, don't the hobbits in the Scouring of the Shire do exactly that? Shoot Wormtongue on impulse? Though, I suppose it could be argued that they panicked because Wormtongue was running away in the book, while that was not the case in the movie.
But it's really difficult to see that in the film, though it must've been their intention. You can't see Grima's hatred of Saruman in the film; this is the first time he's been mistreated. In the books, Grima finally goes off the top after a long line of being abused. In the movies, he's just slapped once. And because in that scene his expression looks like he's trying to repent (after Theoden talks to him), it makes it look like Grima decides to be good and kills the bad guy instead of only killing Saruman because of his hatred for him.
...my biggest problem is that an arrow shouldn't be able to go 500 feet up in the air like that before weakening significantly, if not fall back down, due to gravity.
