Unfortunately for Charlie, Jill was just using him and she killed Charlie herself while claiming that the only thing people will remember is a lone survivor.
Maureen was having an affair with Cotton Weary but it's explained that he was framed by Billy and Stu, who committed the heinous acts because Maureen was having an affair with Billy's dad as well.
In the first film, it's revealed that Maureen was killed a year before the present-day murders in Woodsboro.