Why did the twin die in The Giver?

There is a limit on the population in the society that Jonas and the Giver live in,

The twins were born, and one had to be 'Released' to keep the population within the predetermined limits. Jonas' father weighs the two infants, and injects the smaller of the two with a serum to 'kill' it and sends it away.

Jonas sees this in a recording, and is extremely distressed,

realizing that 'Release' is actually 'Death' and that this could happen to Gabe if

he is unable to sleep peacefully through the night.