Maybe I missed that (I watched it twice), but to me it appeared very much so that he had (except after the intricate swaps, which erased his memory) full agency, and was the one pushing the limits/boundaries of the device.

Of course one can make the point that power corrupts etc., and the Shinigamis are attached to that power. In a way, they embody power and could be seen as the corrupting force, but that is as far as I would take that. At least Riuk was emotionally detached/had no real stake in it. I don’t recall him ever endorsing or driving him to persue his mission. Maybe his subdued admiration of Lights “shenanigans” can be seen as adding fuel to the his (ego) fire, but that also is a defect Light has had before, not instilled by Riuk.