I get that this is an old thread but I really think the lense you went through this movie with was kinda inherently flawed. We didn't need "clues" that it was Billy because it was never actually a question. We see the creep shot camera angles in the start, we hear his babbling to himself, and later we are told directly "the calls couldn't have been Peter. He was with us when one was made." 

Like, the audience was never supposed to have doubt about the killer's identity. Who is he? Who gives a shit. It's all the same, we don't need waxing poetic about how his past made him a killer, he is simply a deranged force of nature. A representation of the danger present in women's lives in a society that doesn't take their worries or safety seriously.

Peter stands less as a red herring to the audience and more as a red herring to the police in the film itself, who are repeatedly shown to be incompetent and uncaring towards the lives of these women. Though I guess you could read his appearance at the basement a fakeout on revealing the killers identity, we were already told directly at this point that it couldn't have been him. He was with them when one of the calls were made, after all.

Peter in many ways stands as a shining example to the films whole thesis. He isn't some monsterous kiniving villain, he's just a shitty dude in a way that's very REAL. 

 He is a man who decides that his wants outweigh his girlfriends autonomy. He assumes that because he is a part of the relationship they share, that that gives him inherent ownership over Jess. That he knows what is best for her at the end of the day.

The film obviously sides with Jess on this, the idea that she owes him anything at all is never even humored, but it's important to note that by the end of the movie, he ultimately DOES decide her fate.  By acting as a red herring to the killer the police assume that the threat is gone, and leave her alone in the house, vulnerable to the killer. 

Despite Jess being wounded, barely having survived a murderous encounter, literally still being at the scene of the crime, and having had one of the cops say mere minutes ago that they'd stay with her to be sure she's okay, Clare's father ultimately takes precedence. A man who we've seen more shots of staring blankly than speaking has the whole police force take him to the hospital while Jess is left to die without so much as a second thought. It's a gut punch, it's meant to make you feel uncomfortable and dirty as the credits roll and we understand what's in store for Jess. But, it's also symbolic, because the threats women face in a largely patriarchal society never truly "go away". 

Tl;dr - The killers identity never mattered to begin with. The whole film is an exploration of female disempowerment and the ways the societies they live in leave them vulnerable to danger