He was actually in that house, ignore the other theories....
I've enjoyed reading all the theories on this board. Many would agree the most intriguing is that he was never actually there, that her narration of the events is just a ghost/soul trapped in internal torment as she imagines her son coming home and missing her and ultimately loving her very much.
Well.....no.
Everything that happens in the movie does in fact happen. He is there, 100%. As he makes his way down the stairs after blowing the candles out in his vision (yet again rejecting faith and belief) he has found nothing there worth cherishing and that's why he said sell it all.
When he reaches for the door you hear her say "Don't go" and he seemingly reacts to it...but don't be so easily fooled here as that is the theme of the story. Remember in the basement when she "touches" his back and reacts as if something did brush against his back? Well in the end he rejects that actually happening, well at least that the spirit of his mother touched him. But she is PRETENDING that he felt her. She is hoping beyond hope that he accepts that touch as his mother and that he too is seeking her. But that's not what happen, she even says as much in the end. It's sad as hell but she wanted that reaction to prove he felt her.....but alas he brushed it off as nothing.
This is the same thing with his final shot in the movie. He thinks he hears something, but to her it's all she could hope for, that he actually heard HER say "Don't go". But he didn't hear that. Whatever he did hear he chose to reject. That's the point of the movie. She is reaching out to him and he keeps rejecting her.
The final shot of the house for sale with unkempt grass and her all lonely in the window is because he did leave that day. He did reject her, fully. When she says "Make believe you came looking for me" when it shows him entering the house many interpret that as she is making it all up in her head. Nah, it's just that she was WANTING that to be why he came. She reached out to him in the movie various ways but he kept rejecting her and belief, so ultimately she was let down big time.
As for the cat demon creature that's a separate thing. It's also real and attacks those with little faith. She lost her faith when she decided her son was the most important thing in the world and put him before God. This resulted in extreme loneliness and despair. Such a wasted life, and she pushed her son away. The creature came for her, and killed her. Though it's not physical, in terms of ripping your guts out, but metaphorically speaking it feeds off of you and you die.
Leon had no faith and was also alone, but had to find strength in his own atheism so to speak. Obviously thru talks with the Doctor we know he's dealt with mental issues because of all this. In the end he finds inner strength to defeat the creature, with no reliance on God or faith. So he looks at the creature as his own inner demon to defeat, and he doesn't correlate the existence of the creature with proof that ghosts are real or that his mother's spirit is reaching out to him. He won the battle his mother lost.
Great movie, just wanted to share my 2 cents since I think the acceptance of the theory that it was all in her "ghost mind" cheapens the film.