>>121184033He founded the cult with Grace but he was clearly more of an acessory.
The cult is entirely based on Grace's viewpoint of how the train works, not Simon. Grace told it to Simon and he believed her, and told it to others. That's not the same thing as creating the cult beliefs together. Along with that, the show implies Simon wouldn't have killed without Grace's influence, which says she drove him to be violent as well.
Simon was indoctrinated, because Grace always knew she was saying these things just to make herself look good. She has access to information Simon does not, and knows deep down that she is only guessing about how the train works. Simon is not basing his understanding on his own experience, but instead being constantly lead on and reinforced into belief systems by Grace. The way Grace acts with the Apex kids we can assume she behaved with Simon, how do you think she got so good? This is also supported by the fact that Grace manipulated kids into stealing before she got on the train.
Grace had an excuse when she was ten maybe but it doesn't hold up when she's older and still continuing these patterns of manipulation and exploitation. We see her displeased when Simon is not killing Lake fast enough for her. She's responsible for being a constant influence/pressure on Simon to be violent and turning him into what he became.
