Why did only Lord Rama kill Ravana?
One important reason goes far back to even before Ravana was born. Ravan was actually one of the two gatekeepers at Lord Vishnu's abode Vaikuntha. Once, they stopped the four Sanat kumaras from entering into Vaikuntha and they got cursed. So they took take birth in demonic families as the Lord had desired:
The Lord replied: O brāhmaṇas, know that the punishment you inflicted on them was originally ordained by Me, and therefore they will fall to a birth in a demoniac family. But they will be firmly united with Me in thought through mental concentration intensified by anger, and they will return to My presence shortly. [SB - 3.16.26]
They later on took birth as Hiranakshya, Hirankashyapu in Satya Yuga, Ravana, Kumbhakrana in Treta Yuga, and Kamsa, Sisupala in Dwapara Yuga. So for their deliverance, Vishnu in His avatars had to kill them. So only Lord Rama Killed Ravana.
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