Saw it in the 90’s, movie was American and possibly from anytime during the 80s. It was a low budget horror and teens going into the woods. Typical setup. The killer seemed to be an old man. At one point, one of the teen girls is strung up in a shed with cuts all over her lips. He then kills her, but this is where my memory gets hazy. Either later on or right at that moment, one of the guys from her group comes in and tackles the burly man. He then grabs the knife (yellow handle, or might have been a box cutter) and sits in the old mans back and stabs him continuously. They then run away. Ending scene is a panning if the woods, and you see the old man, wearing a straw hat, just continuing his daily life.
Another tidbit was that they would show his kills from his POV. The vision looked blurry and blocky, kind of like the the Doom or Duke Nukem video games on the Nintendo 64.