Goodbye Jack: Burning to redemption

Spoiler alert: The Shining is a tragedy, and we would probably be negligent to say otherwise.
At this point, Jack has seriously lost it. He's wandering around The Overlook's halls with a knife in his hand, as he screams to Danny, who had run away, to "come out and take it like a man", just like Jack's father did with him in the past.

Although we could feel and predict Jack's deranged attitude, we were able to witness the love for his son in brief moments of clarity and reason, right before his death. This love outshines everything, and they were possible thanks to Danny's courage, who never doubted his father is a decent man.
"The face in front of him changed. […] Suddenly his daddy was there, looking at him in mortal agony, and a sorrow so great that Danny's heart flamed within his chest. […] "Doc," Jack Torrance said. "Run away. Quick. And remember how much I love you." (55.59)
The Overlook is hurt, just like Jack, who had forgotten the boiler room and that later would explode taking him in the burst. After seeing a good Apollo side of this two faced man,  Jack dies, leaving us like that a mixed-up heart regarding feelings towards him, the readers. It wouldn't be mistaken to say that the sense of great talent and potential of love has been lost and wasted with his departure. But death was inevitable, and it was the only way to find true redemption.
Jack is gone. The Overlook is burning to ashes. Life can be rebuilt for Wendy and Danny, without Dionysius's chaotic temper.
Even though the fact of his insanity, we will miss Jack Torrance.

Becoming Derlbert Grady: ALL WORK AND NO PLAY MAKES JACK A DULL BOY

We are near the end. Jack has completely lost his mind, and all he wants to do is murder his family, or anyone that crosses his path while his attempting to.
All the conversations he had with Delbert Grady, are strongly echoing in his head, and we can see and feel how the shadows had taken Jack's sanity. The suffered love, the constant struggle of daily monsters, became insignificant next to his desires of letting all of his frustrations go as he bashes Wendy's brain all over the floor.
The woman in the bathtub, Grady's shade, the barman, the ball, are recurrent visions that Jack is having now, and because he hasn't seen Wendy or spoken to her in a long time, he is losing the little reason he had. Every time she approaches him, he rejects her, and they have reach that point of uncomfortable silences and heavy breathing.

Suddenly, Danny gets wounded by a figure of the room 217, and Wendy blames Jack. We cannot know who harmed him, but logic tells us to suspect of the lying shepherd. Hell breaks lose. Wendy discovers that all Jack has been writing this time, the major masterpiece, is actually a repetition of the same sentence throughout all those sheets: "ALL WORK AND NO PLAY MAKES JACK A DULL BOY". She then realizes how disturbed her husband is, and also how dangerous the whole situation has turned.

Jack stops pretending. He yells at Wendy telling her to remain calm and let him finish talking, and when she does, he smirkingly looks at her and says "Don't be afraid Wendy, I won't hurt you, I'm just  going to bash your brains in. Bash them right the fuck in." Persecution begins, and Wendy manages to lock Jack in the kitchen, and it's a mystery how he escaped from there. Maybe Grady helped him out, maybe.
As soon as he leaves the kitchen, he determines to kill anyone that would dare to walk in front of him, no matter who it might be. Danny hides, Wendy runs, and Halloran, who had just arrived to the hotel to help the boy and his mother, gets hurt by Jack's axe, fortunately, he survives to later help Danny and Wendy escape.
Jack has been possessed by Grady's spirit. That's for sure.

Mayhem: Jack's mind and The Overlook

At this point of the book, there is no doubt that Jack's mind tries to hide traits and glimpses of darkness. The hotel does not help him at all; within its walls there are soft and whispering voices that want to be heard, that want to take over the new writer's concentration. Drought gets harder as we enter these chapters, as we see Jack drinking nothing while he talks to the invisible barman of the Goldcn Ball Room, we can see how much the isolation is affecting him and that hallutinations are part of the package.
He drifts apart from Wendy and Danny, who is also struggling to deal with The Overlook's charm. Although Jack seems to be working on a masterpiece of play or book, there's something that is not quite matching their new lifestyle, and he begins to act as a child playing hide and seek with someone that is not aware of being part of that game. He ensconces the radio batteries, the tractor keys, and cuts off the telephone cables, because he knows that eventually someone would lose its mind and getting crazy appears to be an attractive idea.
Jack's duality is often seen in the middle of the book, which is where we are right now. There is a part of him trying its best to be a sane, working, loving and decent husband and father; but, on the other hand, we can find the violent, anxious, alcoholic and agressive man who is frustrated with his fate and is about to burst any time soon. It is almost a Dr. Jekyll and Mr. Hyde situation, with the difference of influential factors to the violent behaviors, which are isolation, not drinking booze, a bad relationship with his family, and others.
The monster is looking out the window, and its about to enter to the precious and mystique Overlook.