Yajna never meant animal sacrifice in the sense popularly understood. Yajna in the Vedas meant a noble deed or the highest purifying action. Sometimes Lord says to sacrifice. When we hear the word "sacrifice" we start to think to kill animal. But sacrifice actually means self control. If we control our mind from evil deeds it's a sacrifice, if we die to save our religion it is a sacrifice.
The Vedas is the word of Supreme Lord. Supreme Lord must not say to destroy his creation by killing innocence. Vedas is totally against Human and animal killings. Let we see some hymns for Holy Vedas-* O lord of effulgence! The non-violent Yajna, you prescribe from all sides, is beneficial for all, touches divine proportions and is accepted by noble souls.
-Rigveda 1.1.4
* Do not slaughter this one hoofed animal that neighs and who goes with a speed faster than most of the animals.
-Yajurveda 13.48
* O teeth! You eat rice, you eat barley, you gram and you eat sesame. These cereals are specifically meant for you. Do not kill those who are capable of being fathers and mothers.
-Atharvaveda 6.140.2
* We ought to destroy those who eat cooked as well as uncooked meat, meat involving destruction of males and females, foetus and eggs.
-Atharvaveda 8.6.23
* It is definitely a great sin to kill innocents. Do not kill our cows, horses and people.
-Atharvaveda 10.1.29
* “O human! animals are Aghnya – not to be killed. Protect the animals”
-Yajurveda 1.1
*Protect the animals.
-Yajurveda 6.11
