Theory - Jaqen h'ghar is actually the actress that plays cersei?
The assignment was a test imo. If you look at any of the other folks killed by the faceless men, they're either bad people who deserved it or asked for it (like those who drink the poison water). The actress doesn't qualify for this as she has been chosen simply out of the other actress' jealousy. The actors also talk about 'changing the script', I think this has a lot of relevance regarding the faceless men and their objectives alongside Ayra's own objectives.
Jaqen said it himself that Ayra would not become a faceless man but something else, he still knows that her intentions are to kill for her own reasons which in his view point could be just.
The way the actress talks to Ayra suggests it to, talking about pretending to be someone else and how she is the only one to take any interest in her whereas the other actors don't seem to, or at least not nearly as much so.
Jaqen also says that one way or another a face will be added, I believe this will be that of the Waif's and not Ayra's.
I could be wrong of course but I think that if Jaqen really intended to have Ayra killed , he'd do it himself. I know the he and the Waif mentioned something about an agreement but I think he was doing that to draw her out, show her up as such and probably knows the Ayra is fully capable of winning the fight.