Very few people are not capable of killing in the proper circunstances.
For most, those circunstances would involve severe disturbance due to fear for one's loved ones (the self-evident example is a mother fearing for the safety of her children), self-preservation, hunger or other form of chemical imbalance. It may happen under drug influence and mystical trance as well.
Oh, and then there is war, whose main role in the present days is to banalize and enable killing.
For most, those circunstances would involve severe disturbance due to fear for one's loved ones (the self-evident example is a mother fearing for the safety of her children), self-preservation, hunger or other form of chemical imbalance. It may happen under drug influence and mystical trance as well.
Oh, and then there is war, whose main role in the present days is to banalize and enable killing.