Cow slaughter is an act of the grossest type of ignorance.

 Bhagavad-gita 14.16 P      The Three Modes Of Material Nature
Because of ignorance, people do not perceive that there is a complete state controlled by the Supreme Lord. Every living creature is a son of the Supreme Lord, and He does not tolerate even an ant's being killed. One has to pay for it. So, indulgence in animal killing for the taste of the tongue is the grossest kind of ignorance. A human being has no need to kill animals because God has supplied so many nice things. If one indulges in meat-eating anyway, it is to be understood that he is acting in ignorance and is making his future very dark. Of all kinds of animal killing, the killing of cows is most vicious because the cow gives us all kinds of pleasure by supplying milk. Cow slaughter is an act of the grossest type of ignorance. In the Vedic literature the words gobhih prinita-matsaram indicate that one who, being fully satisfied by milk, is desirous of killing the cow, is in the grossest ignorance.

There is also a prayer in the Vedic literature that states:

 namo brahmanya-devaya go-brahmana-hitaya ca
  jagad-dhitaya krsnaya govindaya namo namah

   "My Lord, You are the well-wisher of the cows and the brahmanas, and You are the well-wisher of the entire human society and world."  The purport is that special mention is given in that prayer for the protection of the cows and the brahmanas. Brahmanas are the symbol of spiritual education, and cows are the symbol of the most valuable food; these two living creatures, the brahmanas and the cows, must be given all protection--that is real advancement of civilization. In modern human society, spiritual knowledge is neglected, and cow killing is encouraged. It is to be understood, then, that human society is advancing in the wrong direction and is clearing the path to its own condemnation. A civilization which guides the citizens to become animals in their next lives is certainly not a human civilization. The present human civilization is, of course, grossly misled by the modes of passion and ignorance. It is a very dangerous age, and all nations should take care to provide the easiest process, Krsna consciousness, to save humanity from the greatest danger.