Anko was born and raised in Konoha. As an apprentice of Orochimaru, she has learned various jutsus, some of them even forbidden (kinjutsu). When she was a kid she must've been extremely talented, because just like Sasuke, Orochimaru gave her the Cursed Seal. When Orochimaru betrayed Konoha and fled from the village, Anko was furious because she was used by Orochimaru and then cast off like trash.
When Orochimaru appears back in Konoha for the first time since he left the village, during the 2nd chuunin exam in the Forest of Death, she felt that it was her job as a former subordinate of Orochimaru to kill him, even at the cost of her own life. Of course, she can't defeat Orochimaru and even regrets coming back alive from the Forest of Death.