Furthermore Aragorn et al specifically saw this conflict as a one-shot dilemma that had to be definitely resolved with the absolute destruction of Sauron. They already knew what negotiated peace with the enemy looked like (Saruman) and were not willing to risk that outcome, or any other outcome that would result in the rise of Sauron again. This is why they risk everything by making a frontal assault on Mordor against overwhelming odds. Killing the messenger / burning bridges is perfectly in line with the character motivations here and actually a point where the original source material fails.
Furthermore Aragorn et al specifically saw this conflict as a one-shot dilemma that had to be definitely resolved with the absolute destruction of Sauron. They already knew what negotiated peace with the enemy looked like (Saruman) and were not willing to risk that outcome, or any other outcome that would result in the rise of Sauron again. This is why they risk everything by making a frontal assault on Mordor against overwhelming odds. Killing the messenger / burning bridges is perfectly in line with the character motivations here and actually a point where the original source material fails.