Three times, when Wes Craven almost blantly told us who killed Tatum, yet most of us chose to ignore it and still try to pretend, it's Billy's look, that's biggest evidence:

Plus lets not forget about killer REENTERING party, while garage on opposite house side is wide open. WHY would Billy get INSIDE, risking accidently facing Sid, or Randy, or anyone, when garage doors are wide open and he is suppoused to appear only FROM OUTSIDES, anyway?

Ps. When Gale goes to van, on Kenny footage from her farewall scene at party, THERE IS NO STU. Interestingly.