Bartholomew Kuma is a former revolutionary and a friend of Monkey D Dragon. So why is he helping the government instead of helping Dragon. We know that he helped Luffy and his crew during the Sabaody Archipelago Incident, and also helped them to keep the Thousand Sunny away from pirates and the world government. But still he offered himself to the government so that they could turn him to a complete Pacifista.
Maybe he was an informant for Dragon but he was too honorable to betray the government bluntly. We know that he was a man of reason when he let Zoro take up all the pain instead of Luffy.
Him becoming Pacifista was entirely for different reasons which we still don't know. He offered himself to be experimented upon but his last wish was to protect thousand sunny which was honoured and his wish was fulfilled.