In Mahabharata yuddha Bheem killed Duryodhan by hitting his thigh as per the suggestion of Lord Krishna. Duryodhan was deciple of Lord Balarama, the mightly elder brother of Lord Krishna. Hence Balarama became furious when he heard this news and rushed in to kill Bheem. However Lord Krishna intervened and told him - "Brother, if you're so worried about the rules being broken": 1. Then where were you when Dusashana pulled Draupadi into the Kuru Sabha by her hair? 2. Where were you when Dusashana tried to take her Saree off? 3. Where were you when Duryodhana was bursting out in laughter on above act of Dusashana? 4. Where were you when Duryodhana denied to give even 5 villages to the Pandavas? If you didn't speak a word on any of the above occasions, then you've no right to kill Bheem now. Moral of the story - you must take a side in a "Dharma Yuddha", can't just sit on the fence and later shout 'injustice'.
