I completely agree. It's a team, Gigi was the initial leader seeking revenge for her sister Amy's (and the 4th girl from the bathroom that took the twins in at first) death. The twins are Chanel #5 (Libby) and Boone and were helping their basically adopted mother with her revenge. So they posed as sorority and fraternity members until the time was right. The people killed were intentional until the killings went too far.
Deaths:
Ms. Bean: obviously had to go, because she was there that night
Any member of kappa or dickie dollars: revenge for that night (Rodger and Dodger's deaths will play into Chanel #5 turning on Gigi and Boone)
Shondell: It was dark and it was Denise's car. Denise was a kappa and has been a target. They either thought it was her or killed her to get back at Denise for being there that night.
Coney: killed so that Pete could have an alibi for owning a red devil costume
So anyway, my theory is that Boone and Chanel #5 worked there way into their fraternitys as a cover, but during that time they both had run ins with Pete. (He met Chanel #5 while stalking Chanel and Boone when he tried to join the DDS) As his hatred for the sororities and frats grew the two twins saw a kindred spirit in the boy. So they brought him in to their plan (much to Gigi's dismay). He was the mascot and gave them the idea for the red devil killer (this is why Gigi does not wear the costume, she did not approve of Pete being a part of the family business at all). And I don't think Pete killed all that much. I think he killed the cop in this past episode (11) and possibly Gigi and/or Boone.
The reason why it's now Pete and Chanel #5 is because of the twins Rodger and Dodger. Just like Boone found Zayday and was shut down, Libby found love with another set of twins. But as they were part of the frat one of them needed to go. Chanel #5 begrudgingly agreed with one of them being killed, but Gigi and Boone decided that revenge took precedent and killed both of the dickie dollars. A heartbroken Libby and a lovelorn Pete who actually has fallen for Grace and so feels Chanel #5's pain, decide to take revenge right back on Gigi and Boone.
Other Things
I've heard a lot of people saying that Chanel #5 can't be the killer because of when the red devil killer is with them in the mall in episode 11. Some people are saying that there were two of them there but I disagree. There were some awkward camera jumps, but I'm pretty sure there was only one red devil there, and that was Pete.
Also, I've heard that the truck scene disproves Chanel #5's involvement. No. A number of things could have happened here. The Red Devil could have been talking to her and was seen by the truck driver. It's not like Hester's story where the man was in regular clothes, the red devil is an notorious serial killer with an obvious and very bright costume. The red devil didn't have to be doing anything menacing at all to scare the truck driver, it was obviously the serial killer. So the red devil didn't need to be doing anything other than sitting there to be stopped by the truck driver. Also, it could have been a ploy to stop the truck. The truck could have had something they needed or the driver could have been someone on their list. But the fact that Chanel #5 isn't dead yet speaks volumes of her involvement.
That's my theory and an idea as to how it happened. If you have specific questions I can provide answers and evidence as to how it works into my theory, but I am almost certain I am right.