Rasputin's a jerk because of his experience in the collapse. He came to understand just how powerless he was in comparison with the Darkness and now has a personal vendetta against it. Because of this, everything else comes second to his hunger for more power. We won't know his future plans until (presumably) the Warmind expansion for Destiny 2. We can guess as to what his next move is, but we don't actually know. Rasputin will be a very large player in events moving forward.