Why did he give her the tape?

In the last several minutes of the documentary portion, where he decides to retrace the path the crew took to get to the Pine Barrens, you can see David is wearing the same clothes after he just killed Shelley, even though the documentary portion claims that it occurs the following day. You can briefly see the plastic he used to wrap her up in the back of his car when he’s talking to the camera while driving. And the final shot of the documentary portion of him in the woods by himself is what we see in the narrative portion.

Now as I said before, the documentary portion shows that March 31st is when he goes into the woods, and April 1st when he goes into Shelley’s to kill her. But as seen in the narrative portion, we see that he killed Shelley before he went to shoot the other segments, further cementing the theme of the movie; the footage should be telling the truth when it really isn’t because it’s edited to fit a narrative that David wants to tell. Same thing with law enforcement with the raw footage from the show; they used what they had to fit a narrative that they wanted in order to convict Jim Suerd for the murders.

Point being, in the documentary section, it seems like he left Shelley to her own devices and occasionally stepped in until she got something. But off camera, in reality, it’s possible he kept checking up on her every day to see how much progress she got to make sure she hasn’t made him yet. Once he decided she got far enough, he broke in and killed her. The segments where we see his face slowly being revealed could just be his work doing that he is editing into his documentary and not Shelley’s.

The documentary ends with the reveal of David being the killer. It not only accomplishes his goal to posthumously exonerate Jim Suerd and incriminate himself, but it supports his thesis that the murders of the Fact or Fiction hosts did nothing but allow everyone around them to profit and expand their careers with it. And David is exploiting that with the documentary that he plans to release to the public. The first 75 minutes is that documentary. The rest that follows is an epilogue of sorts that is separate from the documentary itself.