John Tombs mocking Tree leaving the safety on when she tries to shoot him first comes off as her being inexperienced with a gun, but once she's able to use her knowledge of the incoming blackout to retrieve it and kill him after he's already beaten the crap out of her, it becomes clear that she intentionally left it on in order to intimidate him into attacking her. She needed obvious self-defense injuries to prove to the authorities that she didn't kill an unarmed man but one who was on the verge of taking her own life.
Fridge funny: during the montage Tree goes streaking on the path immediately outside Ryan's and Carter room. Therefore the only place where she could decide to do so was in their room.
The reason why Tree's TV turns itself off and on isn't supernatural or just a horror movie trope thrown-in for creepiness. It's because Lori is in the room and she has the remote. Tree even says, "Where did she put the friggin' remote?"
The reason why the killer seemed to get the jump on Tree so many times is because in at least one cycle, Tree unwittingly tells Lori that she knows someone is out to kill her. And Tree apparently never put Lori on her suspect list to investigate her.
It also makes the scene where Lori completely dismisses Tree's claims that someone is going to kill her later that night by saying, "No one is trying to kill you" much more sinister. It's not that Lori doesn't believe her, it's because she does want to kill her and she's trying to lull her into a false sense of security. Lori didn't bat an eye in trying to convince Tree that she was just being paranoid.
Lori mentions she's the one who's changed Tree's phone ringtone. So Tree either didn't put a passlock on her phone or Lori knows it, and could have easily installed or activated a "find me" feature on her phone— letting her know everywhere Tree is all through the day. Which is why the Babyface killer keeps finding and killing Tree no matter where she is on campus, even when she's investigating a suspect and not at the Sigma House party or heading towards it.
Why isn't Tree ever murdered before the blackout at 9:23pm? Because Lori's working a double shift at Bayview University Hospital. She offered Tree her poisoned cupcake early in the morning, which presumably would have taken some time to work (since once Tree eats it, she died in her sleep) giving Lori at least a tentative alibi. Once Tree refuses the cupcake, Lori has no opportunity to track down and kill Tree until she gets off-shift.
There is actually one early clue of the killer's true identity. One of Tree's murders has the killer use a candle to light up gasoline and blow up the car she's stuck in. Who is the only person in the movie shown using a candle at one point? Although, the candle on the cupcake is red, but the one used to light the trail is green, presumably to leave it as a hint (who do we know has easy access to candles of them shape and would think of using them) but not to make it too obvious.
In the same "candle" clue scene, there is another clue to who the killer is: the killer's sneakers. The scene takes place at night, it is very dark, and the main source of light is the police siren (red and blue flashing light). The killer's shoes, with a white stripe on the bottom, seem to reflect the light and seem to glow in the dark. In what profession would someone might need reflective sneakers due to low light or dark areas with a glow in the dark feature? A hospital employee, specifically a nurse or attending.
Tree not eating Lori's cupcake causes her to suffer so, not just for being a dismissive Alpha Bitch, but it also forces Lori to get, how shall we say, very creative in murdering Tree in increasingly painful ways, since the cupcake was poisoned. When Tree finally Took a Level in Kindness and eats the cupcake, it allows her to figure out that (a) the cupcake was poisoned and (b) Lori was the one who obviously poisoned it and thus has been the killer, leading to Tree finally averting her death.
After Tree apologises to Lori, the latter can be seen frowning as Tree leaves the room. As one of Lori's motives was Tree being a Jerkass, perhaps Lori was feeling guilty about her plan to murder her, or briefly considering sparing her.
There have been other potential options suggested as to how Tree could have fought, evaded, or even killed her stalker throughout the movie. Such as telling the police, setting up a safe room, going to her father, or getting a weapon or a gun. However, there are issues with all of these suggestions which suggests why Tree ended up doing things the way she did.
Even if Tree went to the police, she would have no evidence or even a suspect to link the crime to. So they would be less likely to believe her even if she told them. And even if Tree were to consider getting arrested, she would still have to consider what would happen to her possibility of campus or work life if she managed to beat the time loop. And even though she's a party girl, she mentions she's never been arrested and would likely be afraid it unless the circumstances were dire (such as when she was being pursued by Baby Face in a car and realized that in the heat of the moment, being arrested could save her; she simply lost faith in that method after seeing Baby face had no inhibitions sneaking up on and killing an officer.)
Tree still had major issues with her father that went unresolved until the timeline where she attempted to make things right for everyone with all her observations. It was only after a large amount of self analysis that she was able to even consider that option. And besides hiring someone to protect her, or going to the police, there was not much that Tree's father could have done for her that she could not have done for herself. Besides, she'd sound crazy. And they'd only have a few hours to do it all in. Not to mention the very likely chance that the killer may murder her father right in front of her.
Tree does manage this in the sequel, where she has a close relationship with both her parents. When she asks them to get her far away as possible, they immediately does as she asks. It might have worked if Tree hadn't needed to re-set the loop herself after Carter's death.
Even in the scenario where Tree attempted to barricade herself in her room, it failed. And unlike her room, with only one visible exit or entrance, she likely knows no other place on campus that could possibly serve as a reliable safehouse. And given that, throughout the movie, everyone is a suspect as far as she knows, she can't reliably trust anyone to help her find one without suspicion still being on them. Come on, you suspected Carter too at points.
The weapon one is fairly simple in a few ways. One, Tree is a college girl who has presumably not been proficient with weapons throughout her life, as shown how most of the weapons she utilizes are makeshift or stolen. She has no single weapon which she could reliably utilize in a fight if it came to blows with her knife wielding killer. And given her typical life, Tree is really just your average person when it comes to strength and athletics. Given she has no idea how strong her killer can really be, Tree may not have trusted herself to win a face to face confrontation regardless of the weapon. Additionally, had she considered a firearm, it would have served her little use anyway. Guns are notoriously known for being more dangerous to a nervous newbie than their potential assailant. Even if Tree had been able to obtain a gun within the span of a day, and learned the basics of how to use it, there's nothing saying that she could be proficient enough to shoot at a presumably moving target which she never knows where or when it will come for her.
Even if she evades her killer for the day, if she doesn't know who they are, somebody that determined to murder her is still out there. If she breaks the loop this way, she might not have another chance next time, since it won't be her birthday.
If you get a close look at Carter and Ryan's dorm room, the walls are covered in posters for classic sci-fi and horror movies note the side of the cupboard next to the door has They Live! and Mystery Science Theater 3000, and there's also a Back to the Future poster. No wonder he believes Tree so easily, in more than one loop!
Given Tim's true orientation, did he deliberately take Tree on a terrible first date? As long as he's in a relationship limbo with Tree, he has an excuse not to pursue other girls, who might actually be interested in him.
Tombs starts talking and takes off his mask at the first opportunity - a stark contrast to the previous behavior of the killer. At the moment it can seem like bad writing, but in retrospect it's a big clue that he's not the one behind the mask in the other loops.
There's a visual Shout-Out to Sixteen Candles in the scene where Tree eats the cupcake note it's framed and shot almost exactly like Sixteen Candles' closing scene. As it's her 16th death, and the cupcake has one candle on it each time, this equals how many candles in total?
