Yes, I agree, first encounter was cool, but I can't take her seriously after "We had an agreement: Maron gets you and Dark Brotherhood remains in one piece". Like, did she really expect Maron to leave Dark Brotherhood in peace after we killed his son?
I won't lie, during first walkthrough I killed Cicero. Then I tried to see a whole desk: crazy Cicero attacked Astrid right before we killed Victoria and Emperor's doubler and she sold us to Maron... That means that Cicero knew something and tried to make things different
Plus, don't forget that he managed to hurt badly Visara AND that guy, which is a werewolf, Arbjorn or whatever his name is. Cicero. Alone. With short dagger. Without armor
And don't forget, that Astrid said strait, that she is a leader here, and she doesn't care, that you are a Listener and you are second-in-command after Night Mother and Sithis
And during this quest you will get an ability to summon ghost-assasin and he will say, that Sithis doesn't want us to kill Cicero...
Oh, Astrid, I wish you had more health so I can torture you, not kill in one hit