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Of all the creepy occupational titles in The Giver, we think Jonas' dad's is the most sickening. He's a Nurturer, which means he takes care of babies... and by "takes care of," we mean "occasionally kills them:"
As he continued to watch, the newchild, no longer crying, moved his arms and legs in a jerking motion. Then he went limp. His head fell to the side, his eyes half open. Then he was still.
With an odd, shocked feeling, Jonas recognized the gestures and posture and expression. They were familiar. He had seen them before. But he couldn't remember where. (19.45-46)
What's so hard to understand about Jonas' Father is how he can be so caring and nurturing—look at the pains he takes to help Gabriel, after all—and at the same time be so casual about unnecessary death.
But the fact is, Jonas's Father is the product of his environment. He's been trained to think that death is no biggie, just as Jonas has been trained to understand that it is, in fact, a biggie. When he tells his son that his job at the release is to make the baby "cozy and comfortable," this isn't necessarily at odds with his perception of the whole event. In his opinion, lethal injection probably is cozy and comfortable. With no real understanding of pain, and no sense of the value of a human life, killing a baby is just like checking a box on his list of to-do's.
Still, it's easy to see why Jonas reacts the way that he does. Not only is his Father a murderer, but he's been lying to his son for most of his life. Jonas's rage at his father is also his rage at the entire community, for its ignorance and its disrespect for human life.