It was during an armistice and also at the time when a pledge was given (not to fight) that Shakra (Indra) cut off the head of Namuchi. As he approved of this eternal usage towards the enemy, he did act in this way.
अद्रोहसमयं कृत्वा चिच्छेद नमुचेः शिरः। शक्रः साभिमता तस्य रिपौ वृत्तिः सनातनी॥