I also want to say that if I'm remembering correctly... the BBEG had Multiattack and Matt had either decided at the top of the turn they were taking both their attacks on Molly, or that he might have even rolled both attacks at the same time. And when the first hit dropped Molly to 0 HP, I want to say the second hit was rolled a crit and thus did 2 failed death saves? Something like that? And that either Molly had already had a failed death save and thus now had 3 and died... or on Molly's turn he rolled a death save and failed it, thus passing away. And because the party were all lower than 5th level, no one had Revivify yet to bring him back.
I'm probably missing or misremembering some details, but in either case I want to say Matt didn't intentionally decide to take a second swing on Molly after he was told he fell to 0 HP. It was already prepped to be made even before Talisien told him the result of the first hit.