he did that because he loved her so much and didnt want her to suffer anymore><!she tehn left with itami but before that he put a jutsu on her so that her eyes wouldnt show until age 17.when her eyes started to show they burned and started to bleed (she was accidentally told once that pein was her dad but she didnt believe them since her siblings said she was their full sister.Once she sees her eyes she doesnt want to believe that hes her dad.its later she finds out how much he really loved her mom