Not sure that Supergirl had expected her die (and she seemed to the only Daxamite who succumbed to the lead). Its possible that she was weaked by the fight hence the lead being very lethal very quickly. Will concede that no other Daxamite died because they bugged out very quickly).
It's not about the intent, it's about the result. There's still a direct cause-and-effect link between Supergirl's decision to push the button and Rhea's death. If Rhea had consciously refused to leave despite Supergirl urging her, then that would make her demise the result of her own choice. But that's not how it played out.
I'm struck by how many threads were left open-ended, not just Guardian. Lillian, Henshaw, and Cadmus are still in play. Jeremiah is still missing. Lena still doesn't know Kara's secret identity (which must be practically a record for identity-keeping among leads on a Berlanti show) -- although we got confirmation of what we already suspected, that Cat knows and chooses to keep mum about it. (She was really telegraphing it with that "hero's journey"/"soar over obstacles" speech.)
By the way, I realized when rewatching last week's episode -- this is the second year in a row in which Kara has spent the entire penultimate episode in Supergirl garb. Kinda refreshing, given how much more time this season she's spent in civilian clothes, especially at the DEO.