From: The Initiative: Captain America v.5 #27

The superhuman civil war is over and Captain America is dead. While on his way to be arraigned at a New York City courthouse; Steve Rogers was shot, and died en route to the hospital.  Witnesses to the horrific crime included former partners the Winter Soldier, Falcon and Sharon Carter, Agent 13 of S.H.I.E.L.D.  Working behind the scenes are Cap’s old enemies, the Red Skull – secretly alive inside the body of ex-Soviet general Aleksander Lukin – Doctor Faustus, and mad scientist Arnim Zola.

After losing the man she loves, things become worse for Sharon Carter when she realizes that she has been brainwashed… and that she is the one who delivered the fatal shot that killed Steve Rogers.  Stuck in a downward emotional spiral, Sharon has quit S.H.I.E.L.D.  Meanwhile, as heroes around the country mourn the loss of Captain America and villains step up their schemes, the Winter Soldier goes off the grid, this time with a new mission: to kill Tony Stark, the new director of S.H.I.E.L.D.