Why did Arya stark go blind?

Near the end of Season 5, Arya is well underway in her training as a Faceless Man in the House of Black and White. Like most teenagers, Arya becomes impatient and overconfident in her new skills and overplays her hand. When undertaking one of her training assignments, which was to observe ONLY, she spots Meryn Trant, a Lannister knight who, presumably, killed her beloved first swordsmanship teacher, Syrio Forel. She disobeys her mission objective and assassinates him after an ambush followed by a difficult fight.
Although she avenged her former teacher, she directly disobeyed her current ones, for which there would be consequences. When she enters the House of Black and White to return the face she used for the ambush back on the wall, she is confronted by Jaqen and the Waif who are extremely displeased that Arya directly defied the Many-Faced God by killing someone who was not ordered to kill. As a result, Waif restrains Arya and Jaqen pulls out a vial saying, "Only death can pay for life". However, instead of poisoning Arya, as she herself also expected, Jaqen drinks it and dies. Arya, now grief-stricken, mourns her teacher but shockingly The Waif reveals herself to be Jaqen. Confused, Arya repeatedly removes multiple faces from "Jaqen's" body, until she sees her own face. Arya's eyes fill over with white as she is rendered blind as punishment.