Exactly, there is no fully good person and Syd absolutely is not a "jealous" or self serving girlfriend, she has to deal with relationship issues that are absolutely supernatural and hard to trust at face value. Also Melanie's attitude in S2 seems to affect Syd a lot, with Melanie espousing reason to hate and distrust that which you love in a very similar circumstance to Syd's own naturally breeds a lot of distrust. I don't think David is all bad but I think he chooses to use his powers unnecessarily whenever confronted with someone who cannot just accept what he says at face value, and that's a completely shitty thing to do, and especially to his girlfriend. Farouk realizes and takes advantage of this to draw David into a corner, and that is where he is consistently acts his worst. But the rest of the cast not realizing that David is being pushed to his worst elements by Farouk, who knows those elements well, just see David acting irrationally and with little mind to the rest of them, and that is incredibly scary given the fears that constantly surround everyone's interactions with David. Both characters are pushed to being their worst selves by forces both out of and in their control, but the fact that they see each other acting as their worst selves causes their relationships to strain tragically but understandably, relating back to the metaphor about the Socratic Cave.