Why does Bartholomew Kuma support the world government?
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.