I don't think it will be Arya but she's a suspect. I don't think she really cares about small folk unless they are useful to her. When she and the Hound are in that village by the Vale she says she hates the villagers because they're cowards and has that little episode with the elders daughter where she rips up her soldier doll.
I don't think she'll use the wildfire because it isn't personal enough and Arya wants the pure satisfaction of fulfilling her revenge by sticking the knife in herself and watching the light go out of their eyes. A giant explosion where she wouldn't even see them dying/dead wouldn't achieve that.
My money is on Cersei. Her only reason for living, now that she can't rule outright, is Tomme and Myrcella. Once they are dead I could definitely see her going out suicidal like that to kill whoever she thought was responsible.