[Linnahiril Tinnufinwen]

I believe that Movie-Aragorn's decision to kill the Mouth of Sauron was probably written from an entirely cinamatic stand point. Somewhere along the lines of, "Well, this guy is really annoying and ugly, so lets have Aragorn kill him." Many a non-book audience member would probably cheer.

As far as whether this is honorable or not, I don't think that anyone could ever come to any sort of agreement on this, because each person has their own sense of what they think is right or wrong. Personally, I don't think the film screenwriters even took this into consideration. I originally thought that they may have thrown it in to get some of the audience members to think, "Oh no, Frodo is dead!" But considering that this scene was put into the extended edition of ROtK, this is probably not right.

Another thing that has not been brought up yet is how this killing of the MoS affects the next events in the movie. Although this is entirely from a cinimatic prospective, I must confess that I thought Aragorn's decapitating of the messenger took away from the power of his performance when the Black Gate opens, and he says, "For Frodo," and then runs into battle. In the EE, Aragorn has already killed something by the time he says this. I don't know why, but for me, this previous kill took something (perhaps honor) away from that line.