Honestly, I've been wondering the same since I read 131. Like, did she kill herself too? Did truck-kun claim another victim? Did she do a murder against that ringleader and end up dead by cop or turn the gun on herself? I would REALLY like if that were the thing that happened, but I don't think she's the type of person to do that.
So I'll predict that it was either stress-induced suicide or an unfortunate accident. Suicide makes the most sense because of how her past friend did that and it's one of the first things Elsa tried to do. It also fits her motivations and mindset at the time: Wracked by guilt and grief, at the point in her life where she has to get ALL of her shit together and decide on a course of life. That's prime conditions to snap.
On a related note, Elsa kinda looks like her old friend if she were a redhead. I wonder if that's actually her reincarnated just like Yvonne. Would be pretty poetic.
