One thing is that Rāma is a kṣatriya, thus his dharma is to fight and kill enemies. It is his proper role. Also, he is restoring dharma to the world (as an avatar of Viṣṇu), which Rāvaṇa is upsetting (even if Rāvaṇa also has some good qualities). Part of the issue is that "dharma" has multiple meanings and cannot be adequately conveyed in one word or even several words.
*However,* I agree that Rāma does not seem as perfect as is so often proclaimed.