Brahmastra is considered to be the most powerful ‘astra’ (weapon). In Ramayana, we see that Lord Rama submits Himself to the Brahmastra.
Whereas, in the case of Krishna avatar, when Brahmastra was hurled at the Pandavas, the Lord counters it with His Sudarshana Astra.
When it comes to His own sake, the Lord strictly follows the Dharma. Whereas, when it comes to protect the devotees, the Lord goes to any extent to save the devotee who has surrendered unto Him.
[This was mentioned by Sri Swamiji in His lecture during the Bhagavata Saptaha at Sri Bhuvaneswari Amman Temple in Jafferkhanpet, Chennai between 7th & 13th June, 2009]