As we all know, from all the movies Black Widow turned up in, there has always been secrets about her past. In the Avengers about her wanting to wipe her ledger clean to the new Captain America movie where Robert Redford threatened if she releases Hydra's and S.H.I.E.L.D's, that "the world will know who you really are". Just thinking...So could she really be a double agent or a former Hydra agent that just can't let go of her past? Or was she one of the key reasons why Hydra spreaded vastly within S.H.I.E.L.D? Many questions and more answers needed. I'm going to leave it at this (SPOILERS):
In the end credit scene of Captain America The Winter Soldier, it shows Baron Strucker talking about the twins and how S.H.I.E.L.D found out about Hydra, but how did he get a hold of Loki's staff when Black Widow was the first and last person to have it in the Avengers?(used it when she was trying to close the portal from the aliens). Just food for thought. What do you think?