Rather neat! I liked the tension it gives you with the candles lighting up!
The sound effects seemed very fitting, except for when a ghost was hit, it felt too "magical" and positive. The lack of a background music added to the creepiness.
I did not understand how did I start that frenzy mode that ends with the win situation, is it always at a fixed amount of ghosts taken down? The first time I saw it it seemed to happen right after I shot a common portrait.
Leaving the gun pointing at the middle makes it rather easy to see ghosts appearing, maybe slightly dimming the area of view could counter that strategy. Also, the gun seemed to get wonky if the mouse went out of the portraits area, nothing big though.
I would add some sort of text to the game over because when it ended I watched the blood spilling and thought I had to reload, but just a click seemed to bring me back to action.
All in all, a very nice creepy environment you got there! Congratulations! (Curiously I managed to get exactly 104 ghosts in both runs I won!)