Srimad Bhagavatam,
namo brahmaṇya devaya, go brahmaṇa hitaya ca,
jagad dhitaya krsnaya, govindaya namo namaḥ,
Translation : I salute the supreme truth that is god, I salute krishna the protector of the cow and the brahman too,
From the Bhagvat Gita,
karmaṇaḥ sukṛtasyahuḥ, sattvikam nirmalam phalam,
rajasas tu phalam duḥkham, ajnanam tamasaḥ phalam,
The result of pious action is pure and will give you fruit of peace and goodness.
Action done in greed and passion gives misery, and action performed in the mode of ignorance results in foolishness.
Killing of the cow by a hindu is that act of Tamas that give the fruit of ignorance.
But for some ignorance is Bliss.