Was this whole movie a dream?
If not,
Who stabbed his friend? There's no way he could stab him, clean the blood of the knife and himself and make it back to his bed.
Why didn't he review the footage after filming the guy looking through the glass door?
How did the guy become a detective? Because he clearly had no idea how to be a cop. And who drops a suspect's name in conversation with the victim?
Who the hell films four people just eating dinner?
The answer to all of your questions: This is a bad movie.
I also wondered why they didn't review the footage.
And who the hell makes a film of someone filming four people just eating dinner?
After watching Jeremy Saulnier's Green Room last night, this seems especially lame. GR doesn't make any of the characters do anything stupid...like NOT reviewing footage you took 10 seconds ago. The girls don't even trip or cry!
Maybe he did review it? If he was looking at it from his phone then he couldn't film the same time he's looking at the the video.
The dropping the suspect's name is kind of dumb, but I don't think he otherwise did that much that was unbelievable.
As for the filming, this type of film-making requires some suspension of disbelief. I understand that people don't all like found footage style films, but it is what it is.