It is said in the sastras that by killing animals in a sacrifice, one immediately promotes them to human birth. Similarly, by killing their enemies on a battlefield, the ksatriyas who fight for a right cause are elevated to the heavenly planets
SB Canto 4
All these animals are awaiting your death so that they can avenge the injuries you have inflicted upon them. After you die, they will angrily pierce your body with iron horns.
Nārada Muni wanted to draw King Prācīnabarhiṣat's attention to the excesses of killing animals in sacrifices. It is said in the śāstras that by killing animals in a sacrifice, one immediately promotes them to human birth. Similarly, by killing their enemies on a battlefield, the kṣatriyas who fight for a right cause are elevated to the heavenly planets after death. In Manu-saṁhitā it is stated that it is necessary for a king to execute a murderer so that the murderer will not suffer for his criminal actions in his next life. On the basis of such understanding, Nārada Muni warns the King that the animals killed in sacrifices by the King await him at his death in order to avenge themselves. Nārada Muni is not contradicting himself here. Nārada Muni wanted to convince the King that overindulgence in animal sacrifice is risky because as soon as there is a small discrepancy in the execution of such a sacrifice, the slaughtered animal may not be promoted to a human form of life. Consequently, the person performing sacrifice will be responsible for the death of the animal, just as much as a murderer is responsible for killing another man. When animals are killed in a slaughterhouse, six people connected with the killing are responsible for the murder. The person who gives permission for the killing, the person who kills, the person who helps, the person who purchases the meat, the person who cooks the flesh and the person who eats it, all become entangled in the killing. Nārada Muni wanted to draw the King's attention to this fact. Thus animal-killing is not encouraged even in a sacrifice.