Who are the antagonists in Lowry's The Giver?
The antagonist of any story is the main person or obstacle that stands in the protagonist's way of achieving his/her goal. There isn't one person who gets in Jonas's face and battles with him about not leaving the community, for example. The major conflict is Jonas vs Sameness; therefore, it is Sameness, the system under which everyone lives, that Jonas must conquer. Sameness was started "back and back and back" as the Giver says, by ancestors who don't exist for Jonas anymore (95).
Some might argue that the elders are the antagonists because they are the ones who enforce Sameness and they know better. But the Giver says that they really don't know what they are doing: first, because they are products of the system of Sameness themselves; and second, they don't have the memories to help them make such a critical decision for the community. All the elders know is that the Receiver holds all of the pain and suffering for the community so they all can lead "happier" lives. If they did know the full extent at which Sameness inhibits people from leading more abundant lives, they may have sided with the Giver that Sameness should end. But there's no specific elder or elders who are the villains of the story.
Hence, there isn't anyone to point a finger at to blame or hold accountable for setting up a system of government that takes away people's ability to make choices of preference, euthanizes babies and the elderly, or forces people to take pills to oppress their sensual desires. The system just exists and Jonas must discover what it is exactly in order to know how to break it down. If he succeeds, people would be able to take back their free agency and the ability and freedom to make their own choices.