I forgot, when did that happen? My long-term memory is going... If she thinks at first that Dexter's murders started only with only him meeting Lumen (randomly?) and helping her get revenge - and by extension get revenge for Rita - then she could be cool with it. She could chalk it up to grief and rage, and imagine that if he hadn't stumbled across Lumen at just that point in his life, nothing untoward would ever have happened at all. But there are far hotter potatoes for her to stumble across. Dexter was the Bay Harbor Butcher and let everyone think Doakes was (including his grieving family!!!) Rita died because Dexter was playing footsie with a serial killer (and interfering with the police investigation that could have caught him sooner.) Brian/Rudy was Dexter's brother and only started the relationship with Deb that almost resulted in her death because he was trying to reconnect with Dexter. And the hottest potato of all: Harry's role in training Dexter to be a killer. All those revelations (and I'm sure I'm forgetting a bunch of stuff - for instance, will Deb realize that in retrospect Dexter's marriage to Rita might have started out as a cold-blooded attempt at cover?) are so incendiary that they need to be doled out of the course of a season. It would be fascinating to see the point at which Deb realizes this is too much to take, and Dexter is not a grieving husband who had a momentary lapse in sanity, but a monster who needs to be stopped once and for all.