The affix śatṛ comes after the verb dviṣ when the agent indicated means 'an enemy'.,
The word अमित्र means 'enemy'. As द्विषन् 'enemy', द्विषन्तौ 'two enemies', द्विषन्तः 'enemies'.
Why do we say 'when denoting an enemy'? Observe द्वेष्टि भार्या पतिम् 'the wife hates her husband'.,