One of the mysteries Westworld keeps us guessing at most is the true identity of the Man in Black. He's a curious villain who, in the original 1973 movie, was a rogue host that would kill the guests. In the HBO drama, he retains all his villainy, but seems to be reimagined as an actual guest. Since there's been some evidence of time-hopping cropping up in fan theories on the show, some speculate that the Man in Black is a present-day version of a character viewers see in flashback. So is Logan the Man in Black on Westworld? It's possible.
The popular hypothesis among fans is that William and Logan's storyline is actually a flashback to a past version of Westworld. An older logo for the park accompanies them everywhere they go, and fans know that there's a 30-year-old catastrophic event that befell the park, seriously shaking up its future. It seems like William and Logan are the plot devices for learning what that event was. Some fans believe that William is a younger version of the Man in Black, owing to his romantic interest in Dolores and initial discomfort with Logan's black hat version of Westworld. William wants to be a good guy, but is forced to make bad choices. It stands to reason, then, that he stays so obsessed with the game because he wishes to free himself from the villain he ultimately becomes.
But what if Logan is and has always been the Man in Black? Logan is the one who enters the game with a black hat and chooses the more violent loops to play. He's the one who coaxes William deeper into the game, outside of Sweetwater and into the rawer storylines of Westworld. And, he's always had a sort of disdain for Dolores, seemingly because William is so invested in her. It's possible that Logan-as-the-Man-in-Black continues to be so horrible to Dolores to punish her for having such a strong presence during his trip with William. Perhaps she's somehow responsible for whatever happens to William and Logan harbors some lingering resentment toward her.
Logan, too, is an experienced guest of Westworld and it would make sense why he'd want to explore deeper levels of the game, eventually becoming myopically obsessed with finding the maze at the center. He's much more enthusiastic about the inner workings of Westworld than William is, and I could see him returning to the game again and again for thirty years, "killing" hosts with impunity, to find what he's looking for.