If you read the artifact lore we also have to regain his trust, despite season of the worthy being solely about us gaining his trust.
The end cutscene has been data mined.
I won't spoil but there is a reason that will be revealed as to how Rasputin could ruin the Witness plan. It's not obvious, but if you think about it it makes sense.
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[spoiler]it was so predictable[/spoiler]