Guru Nanak Dev Ji had refused to accept the food of Malik Bhago, even though it was considered pure (saatvik) food, because it had been obtained by oppressing the poor and through exploitation.
Through His miraculous and playful act, Guru Sahib demonstrated this by making the hidden blood in those items visible. To eat the meat of living beings, blood must first be spilled, and force and cruelty are used in the process. Such actions are considered sinful and are the root cause of sin.
Gurbani states:
ਅਸੰਖ ਗਲਵਢ ਹਤਿਆ ਕਮਾਹਿ ॥
ਅਸੰਖ ਪਾਪੀ ਪਾਪੁ ਕਰਿ ਜਾਹਿ ॥੧੮॥