Do animals commit heavier killing karma than humans?

Creating evil karma and the karma of killing is governed by the manas and carried out by the six consciousnesses. However, the manas's own independent mental activity is also a karmic action, called mental karma (karma of mind); it also stores seeds and has retributive results, namely the suffering of birth-and-death in saṃsāra. Among the three kinds of karmic actions—bodily, verbal, and mental—mental karma is the primary one and the heaviest, because bodily actions and verbal actions are directly commanded and controlled by the mental actions of the manas. When the manas gives rise to greed, aversion, and delusion and thereby creates bodily and verbal karma, this is called deliberate action; karmic actions not created through the manas giving rise to greed, aversion, and delusion are called unintentional action. Deliberately creating evil karma brings great and grave sins, possessing intrinsic sin and intrinsic karma; evil karma created unintentionally carries only slight sin, without intrinsic sin or intrinsic karma.

When animals eat one another, it falls between the intentional and the unintentional: it is purely out of hunger, without a mind to kill or a mind of hatred, so the karma created is relatively small and the evil karma relatively light, without intrinsic sin or intrinsic karma—they simply repay the debt in the future when the debt comes due. When one period of evil karma as an animal ends, they may be reborn as humans; if humans then create further evil karma, they will again undergo retribution in the three evil destinies. Thus sentient beings transmigrate through birth and death within the six destinies, and the karmic actions of the human body are the most crucial determining factor.