In the Ramayana, Rama faces Ravana in the battlefield with no armour on His body. Ravana has come in full royal paraphernalia, but Rama has come with no ornaments, or even armour.
Rama knows that His beauty is such that even those who do not want to acknowledge His greatness will be drawn to Him by His beauty and radiance. So He shows His beauty in all its glory to the raging Ravana. Rama's hope is that even at that last moment, Ravana might have a change of heart, repent for his sins, hand over Sita to Him , and seek His pardon.
But Ravana's pride is such that it blinds Him to Rama's beauty. His eyes do not behold the beauty right before their eyes, because the veil of pride covers them.
So he does not make use of this last chance for repentance and forgiveness, but goes ahead with the fight, and the result is that he dies at the hand of the Lord.
The Lord showing His beauty to Ravana was a result of the Lord's mercy, said K.B. Devarajan. He does not want to kill Ravana. He tries to help Ravana extricate himself from the situation in which he finds himself, a situation brought about entirely by Ravana himself.
One would have expected Rama to be unsparing in the case of Ravana. Why should Rama forgive Ravana for the heinous crime of kidnapping Sita and subjecting both Rama and Sita to untold misery? It would have been understandable if He had killed Ravana right away. But He gave Ravana a last chance to save himself. That is the nature of the Lord.
He is always looking for excuses to help us and save us. We are the ones who do not realise this, and squander opportunities which He gives us, for us to seek His forgiveness.
One of the Lord's names in the Vishnu Sahasranama is Sushena, which means ‘He who has a good army.'
The next name in the Sahsranama is Kanakangati, which is a reference to His lovely form. His good looks constitute His army.
He can charm His enemies and avoid killing them. That is what He tries in the case of Ravana, but it does not work , because of Ravana's pride.