Well, definitely I think the extent of visual hallucinations was purely Santanico. But I also believe he could have been having minor aural hallucinations interspersed all through his life (perhaps because of Santanico as well). Or perhaps she preyed upon this preexisting condition. I think Santanico may well have been watching and cataloging Richie for a long time, not just in the five years she made her presence apparent when Seth was in the slammer, but way before that and silently.
Richie’s conception of good/evil and his moral compass were entirely self-created, what with being involved in such an isolating abusive experience as a young child. It’s almost the PUREST (ironically) form, in a way, of a conception of good/ill. Richie knew Seth was being hurt, and Seth and he were always in it -together- so that means Richie was getting hurt. So Richie took to the only means he knew to restore balance, a means that was justified and lauded in the one religion he knew— Hollywood. He committed an act that we can, from our removed positions, even deem admirable and sympathetic as well as something we ourselves wish we could be brave enough to do — something that we usually only see in movies and use that medium to express our desire to be as courageous in drastic situations, even when the lines are not clean-cut. This is very 4th wall-breaking in a way, as this is exactly what Richie the character was doing in reaction to the positive reinforcement his beloved movie-character saints were getting — by virtue of being created as characters at all.
The way I choose to look at before-Richie is by using vamp!Richie, movie!verse Richie and the moments in the series when we can see Seth is visibly relieved by Richie becoming “himself” again.
I think he definitely is sexually fixated, perverse (in the movie he was a registered sex offender and I sort of like to attribute this quality to series Richie as well because this is specifically how Santanico chose to manipulate him and often little physical things he did reinforced this, like bending Jessie over the counter in the pilot and standing up against her backside). I think he might be somewhere on the autism spectrum, and is most definitely highly intelligent, but in no way do I think ‘regular’ Richie was actually certifiable. Richie displays things like social deficits, repetitive behaviors/interests, communicative deficits. He even quite obviously relies on his encyclopaedic film knowledge to “fill in the blanks” of how to talk to/approach people (as evidenced by his very scripted and Noir approach to talking to Kate at the pool — it can be surmised he does this with everyone and uses actual lines from films to get things rolling). I’m curious if there is a movie reference somewhere relating back to the “nice hair” comment he made to Jessie at Benny’s, or if it was just used to display his ineptitude.
I have studied medicine and psychology and I worked in a mental hospital, and regular!Richie definitely would not be treatable for anything as presented on screen, as Seth seemed visibly distraught and SURPRISED (key word) over Richie’s mental dilapidation and often was beside himself at Richie presenting specific traits (disregard for life, suggestions of sexual assault, murder, hearing voices, sociopathy). This tells me regular!Richie didn’t display these things, and had a more level-headed idea of how to behave, after maybe Seth explained it to him. He was shown visibly torn in the bathroom of the Abilene Bank when Santanico was trying to get him to kill Monica— he was fighting it. He also expressed distressed horror when he was turning, “she had a family and you made me kill her! you made me cut her eyes out!" Even if Richie has suppressed desire to hurt/sexually demean women (I believe he probably did), that is entirely different from acting on that desire. Something it seems only Santanico’s influence could get him to do to that extent.
However, in his regular life I think people would absolutely view him as “crazy”, and perhaps his actions would absolutely be in line with what the common incorrect conception of “madness” is, but I don’t think he was unstable or insane so much as he was dependent on Seth in a lot of ways to filter social interaction, emotion, and ostracized both internally and externally because of his aversion to and inability to grasp the un-logic/logic of it himself. Because of his unique mind.
If we take what we got from the series, pre-series Richie designed thirty-five perfect bank heists, worked successfully (alongside Seth) in a unit of other robbers in KC, kept a secret about his father’s death for 10+ years (because he understood it would be perceived as “wrong”), despised Vanessa, worked tirelessly to free Seth from prison. He seemed capable but removed. Richie isn’t quiet because he’s shy or self-doubting, he’s just quiet. It seems like he was entirely sure of his abilities and skills, proud of them, and openly mocked others regarding them (probably partly because of the fact that he was so otherwise ‘useless’ in common society, but still).
Justturned!Richie also seemed extremely clear-headed and in control when discussing the Parker Hotel job with Seth before the labyrinth. This is presumably after all the voices were gone. He was sure minded, confident, clever, and logical. Even in his most deranged state (the pilot ep and the 1x02, 1x03, 1x04) he was shown helplessly trying to use logic and his prior tools of control to make sense of the delirious madness.
I don’t think Richie was ever insane in his regular life — I think all of the elements were there for mental distress but his close relationship with Seth and his high intelligence enabled him not to behave in the way that he behaved through the series under the influence of Santanico. And even if he was slowly losing it, like Seth always feared he might (even in their normal life because of his unique mind and their inability or unwillingness or ignorance towards exploring support/care for it), it can be surmised that Richie hadn’t gone that route before the start of the series. It was Santanico’s push that slid everything down the mountainside from its precarious, but stable, former position.
But yeah, I definitely don’t think Richie was this little wilting misunderstood flower at all. He was always cocky, self-assured, quiet and calculating. He might have kept to himself to avoid problems in social interaction, or out of a plain dislike for it. Seth clearly took over that position in their relationship, and it seems like Richie followed his lead in that specific area. I definitely think he feels like “outside looking in” type sentiment in regard to many regular experiences he probably never got to have (that Seth got in abundance), definitely so in regards to women, but I also don’t think he’d care much about it if it did get it— apart from the feeling of success in knowing he figured out how to get it.
TL;DR guy always had problems, but he wasn’t insane.
And yesss yess yess!!! I really hope we see more flashbacks. I’m hoping they give us a lot of human!Richie and pre-prison Seth flashbacks to make up for noglasses!Richie now that he’s a vamp. Maybe they could explore a past job like Phoenix in tandem with the current plotline.