Question: who has arya killed so far?
Answer: Arya freed Jaqen and the others, one of whom, Rorge, had threatened to sexually assault her. When Arya and the Hound were attacked by Rorge later, she stabbed him in the gut with Needle. Ser Meryn is the guy who killed Arya's sword instructor, Syrio Forel, back in King's Landing in Season 1.