 नियतस्समुदाचारो भक्तिर्भर्तरि चोत्तमा।

यन्न हन्ति दशग्रीवं सा महात्मा कृतागसम्।।
  
 "Ravana is a great soul because, even though he has done harm to her, he is not killed. This is because Sita being a chaste woman of good conduct and deep devotion towards her husband she refrained from killing Ravana as she wanted her husband to do that. 