Maharaja Dasarath, although he was a great devotee of the Lord, but because he was a Ksatriya king, keeping one's promise is inevitable for him. He preferred to banish Ramacandra on the request of his wife on the principle of keeping his promise. In higher states of spiritual life, one can break even promises also, but they are devotees of comparative merit. In the case of Vasudeva, we find that he was spiritually more advanced than Maharaja Dasarath. Vasudeva was also in agreement with Kamsa that he would deliver all his sons to his hand, as soon as the child is born. But in the case of Krishna he broke his promise.
Letter to Satsvarupa, 16 June, 1968