I think I have watched this film more times than any film I own.
I'll keep this short if I can help it, what makes this movie so good to me is:
The Atmosphere.
Slight spoilers at the end.
but the movie is still good every time I watch it.
There's not a lot of action scenes, jump scares or anything like that. Its more of a slow burn, build up, psychological teaser.
There's so much symbolism and subtext going on here that it takes at least 8 watches to catch 72% of it. But this is why I love it.
Every location and room in the film feels, it has a mood to it. The hotel itself is its own character.
Above all this, all the events that take place (minus a few unrealistic things) can very easily happen in real life.
Jacks decent into madness, this particular topic this is my interpretation of what happened. It's important to remember that Jack is on the "clock" so to speak, looking over the hotel is his job.
Murder is and will always be wrong. The isolated human mind creates a condition (cabin fever) in which right and wrong becomes irrelevant. It's more like a manifestation. You won't know what is real or not, essentially you start losing your mind and The hotel acts as a catalyst enhancing the process. Its very good at dismantling reasoning and empathy.
Near the end, You will notice that jack seems in a somewhat "normal" demeanor for someone attempting to murder his family.
To him, this is justified. he's just doing his job. After Wendy decided she was going to leave, which in his mind was a problem, just another problem to "fix". Grady and the other apparitions were reinforceing this sentiment too, which all could of just been in his head. A man losing his mind. The mind is very powerful.
You see. It's hard to comprehend how that could happen, but I see people crash out without all that isolation too so idk. I think the point was to display how very real that can be.