In our species, MizOpinionated, there is a strict proscription against the taking of another's life.
It's older than Hammurabi, older than Cain and Abel, older than Osiris and Seth even. Perhaps it is because life is viewed as an inexplicable gift from the unknown without that we view this act as the most heinous crime there is. In other words, if life is a divine gift, how dare any one of us take it away from another of our kind?
In fact we now view murder as such a serious crime that there is no statute of limitations on bringing the act's perpetrator to justice.
There is also the question of reponsiblity. We believe that each person is responsible for his or her actions, no matter if drugs, booze or whatever cloud the senses.
Perhaps that is why we react so strongly. We are only being human.