I have an idea that I've been thinking about for a long time.
Now we can say that Quinn's flashback from Ep.6 and the intro of Ep.11 are the same event from different perspectives. Earlier we saw it through the eyes of little Quinn who got out of bed and Lana's perspective: Buddy and Rox discuss the movie and the rat that should have been killed in it. We see Rox give Jonathan a poisoned dose of, which Lana later died from. Then from Buddy's words we know that Rox died of an overdose and Quinn saw it, but here's the interesting thing. If we are on Quinn's route, we learn that this happened on the same day that Lana and Jonathan bought the drugs.
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Quinn remembers it very well. What's interesting is that Rox dies with the same symptoms as Lana - convulsions and foaming at the mouth.
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Then Quinn experienced a clouding of her mind from the experience and she cannot remember what exactly Buddy shouted at the dying Rox. Both Lana and Rox died from a fentanyl overdose. And not only Lana, but most likely Rox didn't know that the drugs were poisoned. It's doubtful that the leader of a gang, an experienced drug dealer who regularly uses drugs himself, would do such a stupid thing consciously. Rox is a criminal, but I didn't get the impression of him as a stupid man.
Lana's and Rox's drugs were from the same batch and they were all poisoned - most likely these were Rox's personal supplies and he sold a small dose of his drugs for "a party was a rookie", thinking they were good. And who poisoned the batch? There's no need to think long - it was Buddy. Staging an overdose is a good way to remove the boss and take his place, while causing a minimum of suspicion.
I think Buddy is the culprit in the death of both Lana and Rox.