<P> Meanwhile, Dr. Miranda Bailey (Chandra Wilson) and her patient Mary drag intern Charles Percy, who has been shot, through the hallway but when they get to the elevators, they are off . Bailey freaks out for a moment, then gathers herself and sits with Charles . When he asks if he was dying, she said yes . She tells him she and Mary are going to stay with him and that he isn't alone, with Charles dying soon thereafter . The shooter walks into the OR where Cristina and Jackson are trying to save Derek . Hunt shows up and Meredith tells him what was going on . He looks through the window and says she is doing fine but he'd go in to see what he could do to help . When Hunt goes inside, the audience is shown that Mr. Clark is in the room with a gun to Cristina's head, shouting at her to stop fixing Derek . Cristina and Avery refuse to stop working . </P> <P> Mr. Clark tells Hunt that he'd shoot him first and then Cristina but he really only wanted to kill Derek . He believes this is retribution for his wife's death, which he believes is on Derek's hands . Meredith then comes into the room and tells Mr. Clark to shoot her . She explains that she is Lexie's sister, she is the closest thing Webber has to a daughter, and she is Derek's wife . He turns the gun toward Meredith, but Cristina says Meredith is pregnant . Hunt makes a move toward Mr. Clark and is shot, knocking him unconscious . Cristina and Avery then raise their hands and Avery tells Mr. Clark that Derek will die and he can watch it happen on the monitor . Derek flatlines and Meredith cries almost hysterically . Mr. Clark walks out . </P> <P> Clark later comes across Lexie, who is bringing supplies to help Alex, and when he attempts to shoot her, the recently - arrived SWAT team wounds Clark . This gives time for Lexie to escape . Soon after, Webber walks through the halls of the hospital and finds several bodies . In a room, he finds Mr. Clark, who tells him he'd bought the gun at a superstore and bought extra ammo because it was on sale . Mr. Clark tells Webber he has one bullet left . He reveals that he planned on shooting Webber, then himself . Eventually, after Webber convinces Clark that his wife wouldn't want the shooting and would rather be reunited with him, Clark commits suicide as the SWAT team is directly out the door . </P> <P> Meanwhile, in the operating room, Derek's heartbeat returns to normal, and everyone is relieved . Outside the hospital, Arizona and Callie reinstate their relationship when Callie says she doesn't want to have a baby if it means she can't be with Arizona . Arizona agrees to have a baby because she says that she doesn't want to stop Callie from being an excellent mother . Mark and Lexie show up at the other hospital . Alex is out of surgery . Lexie goes to Alex . Mark watches Lexie take Alex's hand . Meredith takes another look at her positive pregnancy test, and throws it out . </P>

Who does the shooting in grey's anatomy