They said Melanie was horribly disfigured, but never said she died. But I think the "Dorkus" title is because they're going to flashback to that day and show us what really happened, and how everything that's going on now started. I don't really think she'll suddenly be made a consequential character out of nowhere...though on the other hand, she's tall enough for the costume and has a built in excuse for keeping the mask on in private.
What if that whole final scene between Pete and Grace is out of context to throw us off? Maybe Pete is in a rush to go and suddenly nervous around Grace because he's found proof Grace is the killer. Say it's Zayday he's on the phone with, he's warned her because they're the two people who genuinely care about her. For Pete it's "gone far enough" because he's backed into a corner - he doesn't want to out Grace because he loves her, but he also realizes she could turn on him. Admitting he's a murderer is just a bluff.
So, this is assuming Pete is the RD (or A RD) and is in cohoots? Or just whoever may be working with him, for good or ill?
Most likely Possabilities of who Killer Pete is working with (And why that will go horribly wrong)
- Sophia's Daughter - whether it's #1, #3, #5, or Hester, I think this would have happened late in the game, and he wasn't "in" the R-D Posse in the beginning.
- Melanie Dorkus - If he's a realtive of Melanie, he could have come to college undercover to hunt down her attacker.
- Wes - Whether they came late to the killing game or not, the two have developed a friendship a wee bit rapidly. (Of course, if the still in the finale of Wes & Munsch in bed together is accurate, this could go badly)
- Munsch - he was agent all along? Pulling strings in her "Get Rid of Kappa, DMT" plan.
- Grace - this pretty much only works if Grace has multiple personalities. Which I think is likely, honestly.
- Boone - This is a "he knew Boone from earlier in life, but didn't it WAS Boone." This only bubbled up as a possible explination for getting all of Boone's stuff. Heck, maybe Chad confronting Pete was actually a front where Chad was trying to say, "I know what Boone was up to, and I suspect you do too."
Family/Friends of anyone at DDS or KKT - again, back to the investigative journalist thing, if he's been less than honest about his presence at Wallace (and how eager he is to investigate), he could be there as the agent of anyone else who vanished more recently. Based on this, the whole "baby in the bathtub" thing could be a smoke screen for #4 or Dorkus's family hiring him. It would NOT be Zayday if we're talking Pete as a killer. She's the one we've seen on-screen being held hostage. Though - with Zayday switching to the "Kill Munsch" side so quickly, if Pete's cause is vaguely moral, he might have pulled Z to his team. Hrm.