To be honest, the Els/Vi interaction you are referencing here is the scene that cemented Els as "best girl" for me. Relationships are always a competition of some kind in the real world, so saying she is playing a "game" to win the MC's love ultimately says nothing about her.
More importantly though, if anyone thinks that Els is selfish and/or abusive in some way, then let me tell you, as horrible as a "mean girl" can be to a guy, they're always
ten times worse to other women. That's the true gauge for spotting awful female behaviour. Especially if they are malicious enough to see that girl as an obstacle on their way to a man's affection. If Els was putting on an act (and that is a
big if as far as I'm concerned), and she was truly not a nice person underneath, then she would have taken an inexperienced and shy girl like Vi and just eaten her up and spat her out without a second thought. Again, let's consider what we
didn't see in this scene. There was no comment like: "You've grown into such a beautiful young woman, Vi, despite what happened to you." Ouch. We didn't see that type of negging from Els, subconsciously reminding Vi of her scars, and attempting to undercut her self-confidence. Or if you think that's bad, how about: "Before you try to win Valentine's heart, make sure you borrow my razor and get rid of that hair on your chin. Oh...sorry. My bad, that's not facial hair. Oh well, I'm sure he'll get used to it." Double ouch. Now that one is obviously too direct of an insult, but Vi's weakness is clear, and a truly awful person could easily find dozens of ways to try and destroy her through pointing out the unimportant and superficial blemishes she carries on her face and elsewhere.
But what actually happened in this scene? Well, instead we get Els giving Vi what I can only describe as an adult version of a pep talk. It is full of good advice, by encouraging Vi to fight for what she wants, and telling her to share her honest feelings with MC. That's...kind of wholesome. Personally, I saw this as Els supporting Vi in a direct way, because Els wants other women to become independent and confident too. Yes, she was honest about her own intentions to win that contest, but the bits of dialog you refer to as "warnings" to Vi have to be considered in context:
With the first quote, Els is simply saying she isn't going be a pushover. She says she "plays for keeps" only
after encouraging Vi to fight for what she wants, so it makes sense that Els would then say that she was going to be fighting her hardest to win the MC as well. And the context of the second quote ("You don't know what you're getting into") is specifically around Vi not understanding who the MC is, so it is part of a much different subject than warning her about some imagined retribution in general. Indeed, Vi doesn't hear it as a threat at all, and she only responds by standing up for herself and noting the many years she spent growing up with the MC as being more important than Els's shorter time spent with him as a coworker.
And as an aside, because this is important, let's actually consider this question of: "Who knows the MC best?". Els's "I'm the better match" is
not her saying that MC has changed from war. What Els is noting here is that Vi still sees the MC with the blind adoration of a child. To Vi, the MC is a pristine white knight from out of a fairy tale, while Els instead understands him as a full human being, flaws and all. To Vi, the MC is still stuck in that "apple in a young girl's eye" fantasy of an older boy, which Els recognizes – rightfully – as not serving as a good starting point for a healthy long-term relationship. This is a very strong insight from her. By saying: "You don't understand him like I do" she's actually trying to help Vi, and the MC too, by giving Vi some "tough love" and telling her to grow up and see him with a woman's eyes, or if they do end up together, neither of them are going to be happy in the long run when reality inevitably destroys that illusion.
So yeah, I think we came into that conversation with very different perspectives, and it definitely changed how we viewed Els's actions within it quite significantly. I accept that I could be wrong about who she is, though I think that is not likely. However, if future actions do reveal a more dismissive and completely selfish personality, then I'll be willing to admit that her act fooled me. But that would require some major recontextualization through a strong narrative thrust by the story's author to not break verisimilitude for me. Els has had a lot of opportunities and temptations to go full sociopath if that's what she was truly hiding, but – so far at least – she has shown herself through her actions to be a person of integrity, and so I'm running with that for now.
Your ideas about how Els sees her enjoyment of violence (or at least being drawn to violence out of some morbid curiousity) as something truly disgusting about her, and a serious character flaw only the MC could accept, is an interesting facet of her personality I hadn't really considered. This could indeed be the case, but there's a disconnect that happens here. At this point in the story, without any context for her concern, my honest reaction to this revelation from her would be: "Congratulations. You're an adrenaline junkie. Now let's celebrate by having some more wild and rough sex."
Yes, that's a bit of a crass joke about what is probably a touchy subject for her, but right now I can't see why she cares so much about this. You think you're the only person in the world who has twisted thoughts sometimes? Seriously?
I mean, there could be something truly dark behind it all, but I also wonder if it's maybe partly indulgent self-pity that's making it overblown. On the one hand, Els seems too strong-willed to feel sorry for herself over nothing, but on the other hand people can surprise you with their ability to delude themselves.
Actually, this might be a sort of last resort for a smart MC to draw out those secrets from Els. MC: "You know you just sound like you're wallowing in self-pity? Part of me wants to just tell you to snap out of it." Els: "You don't know what I've seen, or what I've been through." MC: "Yeah, I don't. And that's the problem. Until I do, all I can say is to stop feeling sorry for yourself. Because you know what? I'm tired of having to imagine all the horrible memories that would make you put up such huge walls around yourself, all so that I don't 'hurt your feelings'. Look. If you're not ready to talk about it, then I won't make you. But you should understand: it's mentally exhausting for me to live like this."
Well, the MC does have long conversations with his two dead friends regularly. Though I understand what you are saying here, because even those particular delusions (whether they are diagnosed as just auditory hallucinations or outright schizophrenia) are not really presented to the reader as an "illness" that he is concerned about. In the story they just sort of...happen. The imagined conversations are used more as a narrative device to avoid relying on excessive flashbacks than anything else. So I feel it doesn't quite land to us readers as representative of his poor mental health. However, even if those interactions are just meant as symbolic and are not an actual psychological issue that is dealt with directly, it's still clear from them that he has things that are bothering him from his past life.
So I don't think that therapy is out of the question here, though a lot of that depends on getting the right kind of therapist. The MC has been dealing reasonably well with his problems so far, at least to my untrained eye. So as long as he gets counseling from someone who recognizes that he is processing things in a "moving into the future" manner (the more "masculine" approach to healing, which is primarily done through being active and by the creation of new memories and experiences) rather than forcing him into a "reminiscing about the past" solution (the more "feminine" approach, realized through talking and the discussion and understanding of existing memories), then I think he can benefit from some professional insight into his situation. That's why I think the "group chat session" stuff at the Center is not a good fit for him. Like most guys, he just leaves long discussions and meetings mentally exhausted and feeling like both they and he were unproductive. Physical training and fighting have clear and tangible outcomes, which from the story have shown to mean more to his mental well-being than endless talking and words.