the ending for High Tension actually does make sense unfortunately its Alexandre Aja fault for it being confusing.

if you listen to the commentary, Aja talks about how he had many clues and hints in the movie […]

“Joey Tone? haha sorry I’m blocking my sign, its actually Joey Fatone as in ‘Fat One’ there’s nothing tone about me haha”….”here let me move to my left–OH SHIT A SHARK!”