He must have prepared a contingency plan... As his cooperation with Russell was nearing an end, he would pull someone else into his fold to deal with Russell.
But for now, Russell did indeed plan to swallow the sugar-coated bullet offered by the Church.
At least, he would eat the sugar coating. The bullet itself could be sent back.
Because even if the Church truly had the power to eliminate the Monkey-faced Eagle on its own, Russell had to intervene and seize the kill.
—The Monkey-faced Eagle was an extremely important supplement for Russell. Its remains were essential for Russell's Spiritual Energy to reach level ten... or rather, they were a condition for him to become a complete Dusk Seed.
If the Church secretly wiped out the Monkey-faced Eagle, it would actually trouble Russell because he would then have to wait for the emergence of a "new Monkey-faced Eagle"... which, if things went as expected, would be the "Promotion Aggregate".