Mikoto's victims were not random.
No.1 中林 大志 (Nakabayashi Taishi)
> Unemployed, and due to it, became psychologically distressed and ended up homeless.
A person Mikoto would never want to be like: (1) Unemployed, because Mikoto got a career he wants/needs to help support himself and family; (2) Homeless, forcibly hidden and ostracized by society; and (3) Suffering, from the distress of unemployment in a society run by work and financial struggles.
No.2 関根 央 (Sekine Hisatsu)
> High school dropout, a freelancer and does part time jobs, and part of a biker gang.
A person that Mikoto would never want to be like: (1) High school dropout, severely restricts one's social status and possibility of getting work/opportunities; (2) Freelancing/Part-time jobs, unstable source of income and Mikoto values normality/stability; (3) Biker gang member, people associated with being low-life, criminals, or delinquency
No.3 桐島 憂吾 (Kirishima Yugo)
> High school dropout, a freelancer and does part time jobs.
A person that Mikoto would never want to be like: (1) High school dropout, severely restricts one's social status and possibility of getting work/opportunities; (2) Freelancing/Part-time jobs, unstable source of income and Mikoto values normality/stability.
No.4 瀬尾 涼 (Seo Ryo)
> University student with a strong sense of justice that patrolled the area in neighborhood watch activities.
A person that may frustrate Mikoto because if Ryo really did have a strong sense of justice, then why wasn't Ryo doing anything to fix the injustices found in labor systems? Was Ryo selective in his justice, only seeing injustices in physical criminal acts? Or Ryo could have been one of those uptight wannabe-cop type of person that reports people for every little thing.
No.5 川島 真司 (Kawashima Shinji)
> Corporate worker and father with 2 children but his life was centered around his work, often coming home late.
Mikoto's mother divorced his father due to Mikoto's father being too busy at work, ergo not prioritizing home or family life. This victim may have spurred buried trauma and emotions/frustrations Mikoto has towards his father. Also, Shinji being a corporate worker spending most of his time at work may be a type of person that Mikoto does not want to be like: Someone whose life entirely consists of work, something that caused him so much suffering.
No.6 忽那 あゆみ (Kutsuna Ayumi)
> High school student, influencer.
Mikoto has a younger sister in high school who he likely was made to be a parent to. He may hold frustrations towards being parentified and that frustration manifested when he saw the high school girl. Not to mention, being an influencer puts the person who is an influencer at risk of harassment and, as a career, may be stigmatized as not a "real job" and an unstable one at that. Mikoto may hold these concerns towards someone that reminds him of his younger sister in the role of an older brother and parental figure.
No.7 井上 陽太 (Inoue Hinata)
> Host club employee, trains at the gym and is an influencer.
Host club workers represent an overarching issue with Japan's loneliness economy in which affection and human closeness are commodified, manufactured by Japan's systems, and profited from. Mikoto may be frustrated by the transactional idea of relationships that Japanese conventions groom unto its citizens. He may also be frustrated at the emphasis on aesthetics in Japanese society that pressures persons to fit to certain looks and to essentially become a body-product for consumption.
No.8 吉川 佐奈 (Yoshikawa Sana)
> Lounge worker, single mother, leaves her child in the care of grandma.
Mikoto may hold frustrations towards his own single mother who he was likely parentified by, and even subject to emotional incest of. There is also a high chance that his own mother wouldn't have been around much due to raising 2 kids on her own, in which single mothers already face hiring/job discrimination and social stigma that are also applied to her child(ren).
No.9 石橋 龍次郎 (Ishibashi Ryuujirou)
> Director of a startup, frequent drinking, worsening health issues.
Mikoto also wanted to make his own studio, which you could say is a startup, where he's the boss/director/lead of it according to a previous interrogation answer. At the same time, he may hold frustrations towards executives in companies for exploiting workers and seeing them as just another stepping stone to get money for the company especially since Ryuujirou is in charge of the corporate planning department which typically oversees a company's operational efficiency, strategy, finances etc. (Corporate planning could also include strategic planning of hiring low-to-no-experience workers/fresh graduates to pay them little but overwork them in order to save money for the company, and the likes.)
No. 10 日暮 夏子 (Higure Natsuko)
> Widowed, mother, living alone, has a son in university.
Mikoto's frustrations with his own single mother may have led to the death of Natsuko, however there is also the aspect of loneliness that needs to be considered. When Mikoto left for university, he left behind his single mother and then-middle schooler sister but it's clear that he still had responsibility to take care of them based on previous interrogation questions. Additionally, Mikoto being all alone in Tokyo parallels the loneliness of Natsuko.
No. 11 田村 快晴 (Tamura Kaisei)
> Security company worker.
Mikoto may have been subject to abuses by corporal/security/cops before just like how Es was instigating and abusing him in Trial 1 VD. Not to mention, he may be frustrated with those in authority who are unquestioned and permitted to do whatever it is they choose without consequence.
No. 12 名護 敬 (Nago Kei)
> Design company employee, diligent and admired by colleagues.
A design company employee who's diligent and admired by colleagues-- things Mikoto wished he was.
Kei physically looks like Mikoto. If there's anything T3 has made abundantly explicit, it's that Mikoto hated himself so much which led to the suppression of the parts of himself that he does not like.
Every part of himself (except for that part he tries to keep the same, frozen) has been created by the very systemic stratae that has destroyed him-- the normalized antagonistic systems that creates overstressed and dysfunctional persons.
He harbors frustration to all those and, yet, he has become a personification of it. Thus, he kills "himself".
He kills a person that is like himself-- someone he could've been if his environments/workplace/community had just been better and hadn't made him like this.