The Final Terror (1981)

Three young campers set out for what they hope to be a fun-filled weekend but find their plans spoiled by a maniacal killer, a crazy old woman who lurks in the middle of the woods for the innocent teen couples to violently dispatch them. The guide to this group turns out to be the son of the lunatic woman, but why she is killing off his clientele, going for the throat in all cases, remains a not very interesting mystery until the end.