We can no longer use Movie_Buff_Brad's left handed killer theory as part of the truth in this, because if you notice how during Randy's death, the killer is right-handed? False, it was a left-handed killer, because the camera was showing the stabbing through a rearview mirror and in which made it look like a right-handed killer. So, for the rest of his theories that follow, nothing is certain, so don't trust this completely, but it's as accurate as it can get.
PHIL AND MAUREEN: Likely, only Mickey was there, as he was the one who wanted to do the copycat murders. He hid in the bathroom stall, and amusingly lured Phil to the stall. Many assume that the voice is a woman's and that the killer here is actually Mrs. Loomis, but the voice sounds much like Timothy Olyphant, who plays Mickey, and Mrs. Loomis wouldn't have the strength to drive a knife through a wall. With Maureen, Mickey took Phil's jacket and went into the theatre. If it was Mrs Loomis, Maureen would have probably sensed that the person sitting beside her was not a male when she touched 'Phil'. Furthermore, one main part of Mrs Loomis' plan was not getting caught. So she probably wouldn't risk killing people in the middle of a crowded movie theatre with so many witnesses. Seems more like something Mickey would do. The following day in campus during class discussion, Mickey pointed out that the killer obviously patterned himself after two serial killers who had been put on film. Various other clues such as the way Mickey keeps on bringing up how the 'girl' (Maureen) was stabbed seven times.
SIDNEY'S PHONE CALL: Cory Gillis, a student attempting to prank Sidney which had presumably been done numerous times before. It didn't work because Sidney and Hallie had caller ID.
CICI: We know there's two killers because when Cici's friend is talking to "Ted" on the phone there is a killer sneaking in behind Cici without a phone at the same time "Ted" is talking. Mrs. Loomis did the calls, Mickey did the murders. Mickey killed Cici before and then sneaked into the party with Derek as if nothing had happened but tried to keep calm so that no one would think of him as a suspect. He didn't arrive at the party until the scene that followed the murder of Cici. Most likely scenario: Mickey films Cici while Mrs. Loomis stays close to the house (by windows and what not) and does the calls, Mickey sneaks into the house while Mrs. Loomis distracts Cici and her friend, hides in the closet, and jumps out and kills Cici. The scene where the killer jumped out of the closet to kill Cici proves that it was Mickey as you can hear him yell as well as crashing into the furniture. The voice definitely wasn't the voice changer because it was more high-pitched (Mickey) like. Also, Mrs Loomis couldn't have picked up Cici and then had the strength to throw her off the balcony, this was clearly a man. Another reason why she couldn't have killed Cici was because she was the one who just wanted revenge. If not anything, she would only go after those who were involved in Woodsboro as all survivors (except Cotton Weary) were at Stu Macher's home where and when Billy's murder took place. Mrs Loomis would have been more than happy to opt to call Cici than to chase after her.
SIDNEY PARTY ATTACK: It was Mrs Loomis. In the previous scene while investigating Cici's death, Mrs Loomis suddenly tells Gale that she has a deadline, leaves the area and it was then when Mickey and the rest of those from the party were leaving the house to check out what was going on at the Omega Beta Zeta house where Cici was killed. Derek goes in and gets sliced on the arm by the killer, he survived with only minor injuries, so the killer didn't mean for him to be killed either. It seems obvious that the reason Mrs Loomis didn't want to kill him was because she was behind the mask, trying to start a plan to get Sidney so scared that she can't even think straight. Why would Loomis want to kill Derek anyway? By only slashing his arm, everyone would suspect that this was done to make it look like he was wounded fighting off the killer but was actually doing it to divert suspicion from himself. Like what Billy and Stu had planned in the first film, to make it seem as if they were attacked by the killer but survived. Also, knowing that Dewey and Sidney was arriving to save Derek, it would be sensible for the injured Mrs Loomis to get out of there as soon as possible before anyone else catches her, like the other two sorority sisters. It couldn't have been Mickey as he was with Hallie and Randy at the time and the chance of him slipping away without being noticed is impossible. If Mickey had been behind the mask, both Randy and Hallie would have got suspicious and he would have given himself away, and if it were him behind the mask, he probably wouldn't hesitate to kill Derek while running into him.
SIDNEY THEATER ATTACK: This was Mickey. You can tell because after Sidney's performance, Derek said that Mickey couldn't pick up Sidney because he had to edit a film, Derek and Mickey had swapped positions. So it's logical that Mickey told Derek that so he would have the opportunity to attack Sidney. Also, so that he could have an alibi. Mrs Loomis does not have access into the campus.
RANDY: Mrs. Loomis clearly admits at the end that she killed him when she says, "Randy spoke poorly of Billy so I got a little knife happy".
LIBRARY SCARE: Mickey, while Mrs. Loomis was off butchering Randy. Mrs Loomis wasn't a student at Windsor College so therefore does not have access to the library computers.
DEWEY AND GALE ATTACK: Both were there. Either one was in the projection room, bailed, and popped up behind Gale, or one was in the projection room and a separate killer pops up behind Gale. The one who pops up behind Gale and later attacks Dewey is most likely Mrs. Loomis, as Mickey would be sent to tail Sidney, Hallie, and the cops.
TWO COPS: Gale's locked in the room and Mickey and Mrs. Loomis can't get to her, and they know Sidney is leaving town, so Loomis sends Mickey like a good boy to go stop her. No way in hell Mrs. Loomis would risk going up against two armed cops, let alone kill anyone besides Sidney and Gale since she was the killer who only wanted 'good old fashioned revenge.' Also, when Mickey reveals himself as one of the killers, you see a large cut on his forehead which indicates that he was the one driving the cop car and then crashing it, killing Officer Richards, and knocking himself unconscious.
HALLIE: Sidney and Hallie get out of the car, Mickey uses those amazing sneaking powers that he showed us in Cici's death scene and slips out of the car, runs behind that wall thing, and jumps out behind Hallie. There was only one killer at the cop car scene and as we have already established, that killer was Mickey.
DEREK: Mickey, obviously.