Simon is mostly known infamously as the Villain of this season and a murderer to boot. He seems to be very insistent about his own opinions and beliefs. Going so far as to outright deny other peoples attempts at correcting him.
Due to this he is seen as narrowed-minded and egotistical, outwardly hating being wrong and giving people this intellectual leverage over him but internally rethinking all of his views. His inferiority complex I mean pride- is what I think to be one of the major factors that causes his character regression.
His pride (No, not the group of lions) is shown throughout the season by holding his ground. Most people interpret it as him being close minded at adamant to his own beliefs but I feel like the beliefs are only the basis to his inferiority.
An example, his own hatred towards Nulls. This can be quite a weak link to bring up as everyone in the Apex has this belief. However, the way he continuously reacts to them despite them showing humanity to him as compared to Grace who has gradually seen the other side of them and starts to understand and develop meaningful relationships with them-
Simon reacts negatively the nicer the nulls are to him.(Tuba’s death)
I interpret this as his own damaged pride, he hates being wrong and the more he is proven wrong the more angry he would get. This fear of being wrong could stem from past experiences of being the butt of arguments and made a fool out of it.
Him experiencing these feelings brings himself to adopt this mentality that other people are perceiving him as an idiot or a fool, causing him to try to justify his beliefs as much as he can in order to victimize himself into thinking that everyone is against him and he is the only one that is right to boost his self-esteem which causes him to adopt this narcissistic and manipulative persona to hide his own weaknesses.
(Simon looks away from Grace displaying emotion, he would be more susceptible to mirroring the reactions of his closest friend and he does not want to show weakness.)
Due to his narcissism, he also displays what the viewers might perceive to be a lack of empathy. He does not display much compassion and can be outright apathetic to other people's circumstances, Such as his dismissal to Hazel bringing along Tuba despite her looking clearly distressed when they said she couldn't bring her and completely ignores her feelings about the subject.
He perhaps reacts this way due to his own internalized hatred of himself and displaying compassion would be showing a less hardened in his eyes as he is perhaps taught from a young age that being sensitive or vulnerable is showing weakness and he does not want people to think less of him.
Him being unempathetic also stems from his ego and beliefs as he usually displays this attribute towards Nulls specifically throughout the show which is because if he was indeed wrong about Nulls, it could cause him to start questioning his own feelings about them which is something he does not want to think about as it conflicts with his own victim complex.
Due to him not wanting to be more exposed emotionally, he lashes out in anger. Anger at himself that he projects onto his relationships with others as he is unable to understand and handle his emotions effectively, this can be caused by not having a stable guardian like figure to help him through such issues.
Simon is also shown to outwardly control his anger by kicking and punching objects.
This is shown to be counter productive as it does not allow the person to understand where their anger is coming from and only prides a temporary outlet for his aggression.
His aggressive personality can also be shown in his interest with making war replicas/models.
This can be taken as him being someone who likes being in control and feels like conflict is the only way to progress which can be found in his own toxic relationship with grace where he tries to control her, not knowing that she is her own person which ties into his lack of empathy. This trait is especially shown almost at the end of the series where he commands the Apex like a leader and donning a military general like coat.
His inability to cope with his own emotions and expressing them outwardly to others is the main reason for his attachment issues.
His own paranoia and lack of trust in Grace by invading her memories when she doesn't tell him that Hazel is a Null which shows us how Grace knows his aggressiveness and acts very secretive around him as she predicts he is going to blow up when he finds out.
Him finding this out himself causes even more hurt as he is hit with the realization the only person that he thinks that ever cares about him has betrayed him. He is alone now, a reminder of the time when Samantha left him behind.
And it angers him and forces him to project his sadness as vengeance against others such as manipulating grace and using her memories against her, causing her to be stuck in her own memories as Simon just walks away.
Put yourself in Simon's shoes, you are stuck in an unfamiliar environment,
no idea where to go and the only parental figure you trusted in this whole place was just using you for her own self interest and
your closest friend seems to be tip toeing around you.
You also feel like everyone is out to get you and that your flawed beliefs are wrong but it was the basis of keeping you sane throughout.
Not wanting to believe that any of these things happened to you, you disregard all emotion and lash out at others because you don't want to feel that any of these things are real.
It sounds like a fever dream doesn't it?
Congratulations, you are basically in denial.
I would say the beauty of Simon's portrayal in season three would definitely be the lack of context behind his actions. He is very easy to dismiss as a one sided evil character. (I don't deny it, he is one.) Due to this we don't really see him as a fleshed out individual and he feels like the Tom Riddle of the season.(slowly devolving into a villain.) His own deterioration into this ultimate descent to becoming a murderer and attempting to murder his childhood friend; the instances to solidify his own inferiority complex (the mentality of "Grace already perceives me as an evil character, what use is there to better myself when it's already too late?") Which is what I think to be the cause of his inevitable demise.
I doubt it is going to happen but I do wish that the next book will cover some parts of Simon's past. He isn't a redeemable character, and never has been. He is someone molded by his own upbringing that we do not see, the most integral parts of his development. Many people have made the comparison that Simon had just as many chances as Grace, if not more to redeem himself.
I feel like he does not need chances, he needs to build himself from the ground up and truly look deep into himself and figure out why is he acting this way. None of these chances provide him with that opportunity and this is where him and Grace deviate.
The train has improved Grace and supported much of her character development as shown in the series where he becomes more empathetic, more trusting and open to others such as Hazel and confronting her own demons in her own mind.
She also finally manages to stick up for herself after the emotional roller coaster Simon put her through.
Simon on the other hand, uses this opportunity on the train to cause much destruction and chaos. He has a disconnect with the plane of reality inside the train as opposed to real life and this stunts his development. In my opinion this would be due to the faults of the train and the limitations of the amount of people that it can help. Yes, some people get off the train and become better. People like Tulip, Jessie and Lake. But we don't really see the people that the system has failed. This is where Simon comes in, he is the very person that the train has failed and it shows how counter intuitive it is with the limited opportunities and the disregard for the different circumstances of the people in it, bundling them together.
I doubt Simon will be the first, or the last passenger to be ruined by the whole system. I do like how his character gives us an insight into something not many of us have thought of before that is; how can a train be designed to be so perfect that it can help every single person on board? It is impossible. Even Artificial Super Intelligence like One-One would have trouble doing so. Which can represent how people can only change if they want to, and if they genuinely don't they have to be stuck the feedback loop of hell.
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Big thanks to my homeboy @useful-crew-too for supplying all of the screenshots for this analysis