You lose health while pummelling Sick Simon to death; a perfect metaphor for being Driven to Suicide.
Using one of the syringes to recover health showcases an animation where Simon pulls his sleeve back and injects himself. Look closely and you'll see cut marks on his wrist. Simon has been self harming.
The apartment complex being so frustrating to navigate, with so many barricades in the stairs and an almost useless elevator. They make sense after you consider this is how a person in a wheelchair, with so much hate and guilt inside them would feel about them.
The rather janky movement in the game, especially with the platforming in the game could possibly represent Simon attempting to remember how his character would walk, especially after being paralyzed but being unable to fully remember because of his depression and other issues.
Considering that Simon goes around with various weapons, killing monsters in the game that represent various negative aspects of his mental health, but also represent other people that he met in his life and that in the bad endings, he can kill either Sophie or Dr. Purnell before offing himself, with the worst having him wish he would've killed far more... it's very likely that Simon harbors some homicidal tendencies, and may have gone on a killing spree if the car crash hadn't crippled him.
Simon is disabled and uses a wheelchair. How could he kill the doctor and Sophie? The sad ending (Simon killed Sophie and then himself) can be explained somewhat. Judging by the happy ending, Sophie is kind and thoughtful enough to visit Simon in the mental hospital despite having many reasons not to (she is not his girlfriend anymore, he murdered two police officers and is mentally unstable, doctors still think her visits are too destabilizing for Simon and that it hinders his progress). Maybe she visited to help care for Simon and one day was killed by him. However, in the very bad ending (Simon kills the doctor and Sophie and kills himself) it is quite out of the question. As discussed above, Simon killing Sophie would be possible. But the doctor was found dead in his apartment. That would mean Simon visited the doctor's apartment, killed him, and returned home. He could’ve gotten caught if he’d shot in the apartment building, so he had to do something else. How easy could it be for a wheelchair user?
In practice, there have been multiple violent criminals in wheelchairs. Also, the apartment could’ve had an elevator, the gun could’ve been held through a pillow to muffle it or someone could’ve thought it was a firework, and cop response time varies depending on the amount of incidents in a given area as well as the distance of the station from the incident.
