Normally I would agree about killing an unarmed messenger. But this guy - if you can call him a guy- this thing, was going WAY beyond of parlay, gloating in their cruel (supposed) murder of Frodo, and then singles out and starts defying Aragorn (?) Like, what? At that point, his words demanding swift retribution, and I wouldn't expect anyone to abide by the of treaty in the face of such depraved disrespect; as Madril told Faramir: "Your life will be forfeit."