Though Ravana was killed by Ram ( Vishnu's dashavatar), but as a fact, Ravana was a follower of Vishnu. Due to a curse from Sanath kumaras and subsequently according to a remedy suggested by vishnu, gatekeepers of Vishnu, Jaya and Vijaya born as rakshashas in three births and killed by Vishnu. Have a look at them who were they in their three births:
Hiranyakashyap- killed by Narsingh Avatar. Ravana- killed by Ram avatar. Shishupala- killed by Krishna avatar.