Why unnecessary killing is harmful in the model of reincarnation

Under karma yoga, we're all conscious beings that are born over and over again, gradually increasing our level of consciousness each time. In each lifetime, whether we're a plant, an ant, spend our lives on mindless hedonism or dedicated spiritual pursuit, we are each still conscious. Our lifetime is a sandbox within which we hope to raise our consciousness, and we should give our energy to others to help them experience this life, perhaps even helping to raise their consciousness as a side effect.

By extension, by killing a conscious being unnecessarily, whether a tree, a mosquito, an animal or a human, we are depriving it of the opportunity to learn the rest of the lessons in its lifetime. Accidents happen, of course, and so does the natural energy exchange of the food chain. We may accidentally harvest too many crops which spoil, or kill an animal where we could have otherwise planned better for our nourishment. But to kill without accident, whether in wilful ignorance or astute cold blood, is to commit that conscious being to restart its life elsewhere, and delay its conscious progression by a lifetime (or perhaps many more).

In this way, we might also view some deaths (such as those from terminal illness) as a merciful escape from a life of suffering devoid of many opportunities to learn. This is not to say that terminal cancer patients deserve to die, of course. But if we understand the dying process as including a choice - at a spiritual level - of whether to continue living in this life, perhaps death is a kindness imparted to conscious beings that have acquired too much damage in this life to make meaningful progress towards their enlightenment.