It seemed pretty clear he'd "turned" at that moment Snokes brutally chews him out, basically calls him a whiney little p*ssy failure and not worthy of Grandpappy and all that jazz.
It stands to reason he didn't just only decide to take out Snoke and claim the First Order as his own right there in the moment when Snoke was torturing Rey. He really seems to have focus & purpose in the scenes before that, leading up to it. It's not like he's genuinely trying to find some middle-ground between Rey/Luke's outlook and his, his goal is to sucker-punch murder Snoke, turn Rey darkside, and rule the galaxy as evil marauding Vader-legacy buddies.
Meaning yeah, he's basically all-in on the darkside thing all through those linked visions/conversations preceding it. Not like he can tell Rey that though.
Rey might have "sensed the good in him", sure, but unlike Luke with Vader she was wrong.