Snoke didn't brainwash/seduce Ben Solo to the Dark Side. Instead, he developed an alternate personality, planted it in Ben's mind, and nurtured it until it took over. The "Ben Solo" personality still exists, and is trying to take back his body.

The new personality, "Kylo Ren," has no idea that there is a second personality in his mind. He has Ben's childhood memories, albeit twisted to suit his state of mind, so there is no (obvious) indication of his take-over. When he feels the Light Side calling to him, it's actually Ben Solo attempt to reassert control.