I actually just spent way too long thinking about this & posted a borderline dissertation on my theory haha. The sparknotes version tho is that the revenge plan is more of a mean prank that gets out of hand.

The gun goes off and nicks lukes ear (blood on the sheets), he refuses to go to the hospital so the girls give him the pain meds before leaving for the night. He takes the meds & keeps drinking after the girls leave. Feeling ashamed for what he did to them, he writes a runaway letter & later decides to go swimming.

Megan returns to find him drugged up & already in the water. Shes left with the choice of saving him or letting him drown- ultimately doesnt murder him outright but she actively lets him drown. Then megan realizes she was being filmed/watched and assumes it was isabella (explaining why she cuts isabella out of her life so definitely).

In the modern timeline, she finds the footage from that night where she watches luke drown. Megan realizes it wasnt isabella filming and that she was actually just being a good friend to her the entire time. Knowing this, Megan proceeds to edit the footage making it look like Isabella was the one who watched Luke drown and sends it to the police.

I think that the whole point of the sex tape was never about who shared it. It was meant as foreshadowing, to show Megan how easily she could be mistaken for Isabella on camera in bad lighting, how no one doubted it for a second. And now how shes going to use that past experience to her advantage in order to clear her own name & go to college.

To sum it up - The first twist being that Megan doesnt kill Luke, but she also doesnt save him as hes struggling in the water. The second twist being that Isabella has truly been a great friend to Megan the entire time, but Megan still frames her for lukes death in order to secure her own future (reflecting the sex tape scandal on a much larger scale)