The short answer here is that evil exists because human beings create it. According to this doctrine, nothing in the world is either inherently good or evil - we see things or events as evil or good. Well, because they will suffer and die if he does his duty. In essence, evil is an illusion or artifice created by human ignorance about the true nature of reality. All sacrifice is born in action - this is the way of Brahman.
Brahman acts continually without attachment to the consequences of the act.
Krishna instructs Arjuna that a sacrifice in knowledge is superior to a sacrifice with material things. One whose knowledge is complete should not disturb the dull-witted whose knowledge is incomplete. These passages seem to imply that in order to properly perform a sacrifice, we must know something essential about performing the act - but it is just this that he enjoins the Yogin to keep from us. I suppose just watching the holy men perform the rituals will be enough to learn the surface procedures - but there seems to be much more to the notion of sacrifice than this. Much has been written about Yoga, and of the different ways of following the Yogic tradition.