My take on the whole Venus thing is that the Vex are attempting to turn Venus into a Vex processor or computer or something, in order to write themselves into the Universe like the Grimoire says. They aren't trying to prepare it for Rasputin or anything, they are just running their usual protocols. Now, what Rasputin is trying to do is achieve what he was built for (as described by him in the Ghost Fragment: Mysteries), which is to WIN. That's all he cares about. He even references "her" from the Black Garden, which leads me to believe the Vex actually worship, or at least did until the Darkness kicked her out. Which would also explain why Atheon's name means something about going against God, which could mean he is rebelling against the Darkness as the Vex's new adopted god in order to try to return to his goddess. Now, when Rasputin mentions "her," he talks about how she was stronger than he was because she didn't have to carry the burden of billions of people to protect. So, in order to WIN, he cast off his burden, which was mankind. After doing that, he began to work towards doing everything he could to complete his goal, and he figured out a way to try and integrate himself into the Vex system on Venus, or at least to exert some level of control over it. However, he hasn't managed to accomplish this yet, so he is still trying. That's why he pounced on the opportunity to get into the systems at the Collective after you opened them up for him. Now, what he plans on doing after that, I have no clue. But I do think that he plans on waging an all out war on the Darkness using every ounce of lost human technology he can take control of, the Vex's systems, and anything else within his grasp. But this time, because of his isolation and his possible rampancy have both caused him to come to the conclusion that to achieve this goal of winning, he must ignore human lives in order to accomplish what he was made for. He sees it as the only way to accomplish his programming.