Lighting, man! Horror is all about lighting.
In general, I would have liked to see a lot more fill lighting. You depended on ambient lighting (mostly practicals), rather than using good lights with fill or even simple bounce to create depth and dimensionality. Without fill, everything looks flat. And video needs adequate lighting, otherwise it just looks gloomy (and I do realize that YouTube's compression darkens up your work somewhat, but the lighting issue is still true). So some of your shots are just way too dark.
Also, recording audio in a "live" room where there's lots of echo requires lavaliers. No way around it. Some of your dialogue is either not crisp, or is simply lost in the audio mishmash.
That exterior shot at 4:54 is rushed, dark, and makes no sense. I see you're trying to make a passage of time, but it's very clunky.
There's no suspense around Den being the killer -- he's clearly crazy form the outset. What would have been a surprise is if he wasn't the killer. Ever hear of a "red herring"? What happened to the cops after they were called the first time? The ending is unsatisfying -- two people locked in the bathroom, one of whom has a phone to call for help. A fundamental horror rule these days is to figure out how to get rid of or disable characters' cell phones!
All this aside, you have a good sense of shot progression and editing, though you seem afraid of closeups -- the entire film is basically wides and mediums. Horror film also requires judicious use of closeups to create inner tension and for reveals.
A good first attempt. I can see you've learned a few things, but still have some lessons to gain through experience. Keep shooting!