I think that one major theme in the book comes down to the role of the individual and their place in society. I believe that Jonas' initial acceptance of his place in the social order has to be contrasted with what comes next, in terms of fully understanding his role outside of it and the moral and ethical obligation to distance himself from it. The evolution of Jonas' character is done with a complete understanding of the thematic importance of the individual vs. society. Another theme in Lowry's work is the presence of difference in society. For Jonas, there is an initial embrace of homogeneity and a complete sense of "sameness." There is a desire to be "the same." Once Jonas works with the Giver, he understands the horror and the shallow brutality in accepting a life of banality without questioning it. He also understands how the pretext of "sameness" can be used to carry out horrific actions, such as releasing. It is through the understanding of how society needs to be changed that Jonas comprehends the dangers of being the same and the need to be different.
There is no perfect world. People make bad decisions.