Gary Clark was the shooter in the final episode of season 6. His reason to do such a horrible action was that his wife, Alison Clark, was taken off life support after she had a stroke under a surgery to remove her cancer. At the time, Alison and Clark had been married for 32 years, and was off course very upset when he got the message.
He has planed this action ever since his wife died. He had bought a gun, and a very large amount of ammunition.
When he arrived at the hospital, he was very upset. He asked many persons to lead him to Derek’s office. After being dismissed several times, he went into a supply closet, where he asked Reed Adamson to help him. When she answered she was too busy, he shot her in the forehead, killing her instantly.
Right after on, Alex Karev came into the supply closet, and Clark shot him in his lower chest.
In the elevato, Clark met Christina Yang, who gave him the directions to Derek’s office, not knowing why.
On his way to Derek’s office, Clark killed Vivian(last name unknown) and Charles Pierce.
When he finally found Shepred, he shoot him in the chest. April Kepner came out of the office, and Clark Aimee at her. She manage to talk herself out of it.
He then went to the OR, where Christina was operating on Derek. Clark asked Christina to stop, but she refused. Owen Hunt tried to charge him, but Clark shot him.
Avery Jackson then told Christina to stop operating and disconnectoderm his leads. Clark left the OR to believe Derek was dead. Once he left, Christina and Avery stared operating again.
He then went to the room where his wife died when he realised he only had one bullet left. His original plan was to first shoot Richard Webber, and then himself. He ended up with shooting himself.