They figured out who was killing everyone, so why did the ghost/monster show up and close her laptop in the end? I tried to find what I could but I just can't find anything.
Please try to re-watch last 15 minutes. You'll definitely understand.
Laura (the ghost) is a best friend of Blaire. Since beginning it's kept suspense who uploaded the video which lead to Laura's suicide. Laura became ghost and started killing the group of friends who are directly and indirectly responsible for Laura's suicide.
At last, in the end, it is shown that
Blaire is the one who actually filmed the video when Laura was in the most awkward situation and uploaded it for fun. This later leads to Laura's suicide. Laura kills them one by one and in the end she came to Blaire to take revenge. That is the scene where you mentioned that Laura appears as ghost when laptop door is closed and post-credits roll. Probably she killed her (Blaier) too.
As per my understanding initially Laura's ghost didnt kill Blaire but wanted her to suffer the guilt when she posted the video and people started abusing Blaire for what she did; however, they kept it an open end with the ghost closing the laptop indicating that the game is over or maybe killing her.
Not exactly I think the point of the ending is that even until the end she would never confess to her crimes even though Blaire new the whole time so she wasn't able to save herself or anyone for that matter. I think the lesson here is to take responsibly for your actions because it's one thing to admit to your crimes, live with the shame and reflect on it but to not own up to it think your crimes will never get out throw it under the rug then say you're sorry. Only after the fact you realise that the person knew all along that she just wanted an honest confession but was never going to receive it. Yes, the movie had to end that way.