You're watching children kill because they have no choice, and the show refuses to look away from what that actually costs them. Triela isn't heroic, she's a broken thing that learned to enjoy breaking other things, and the show is clear that this is damage, not development. The real horror isn't the cybernetics or the missions; it's that these kids are so thoroughly owned by the system that even their small moments of connection feel like programs running their course.
A covert government agency transforms broken children into biological weapons, grafting cybernetic enhancements onto their mutilated bodies and training them as assassins. These cyborg girls carry out wet work against terrorist organizations, their humanity systematized away through conditioning and surgery. Triela stands among them as the most lethal operative, a weapon barely contained in human form, driven by an almost feral hunger to destroy her targets completely. Her opposite number emerges in Pinocchio, a boy-shaped instrument of death created by the FRF, the terrorist faction locked in escalating conflict with the agency. Where Triela burns with volatile intensity, Pinocchio moves with calculated precision, his blade an extension of calculated purpose rather than passion. Both are products of the same machinery that grinds human beings into killing tools. What little memory they retain of their former lives only sharpens the question haunting them: can something remade into a weapon ever become anything else. The series watches these shattered minds collide across a gray landscape where good and evil have already been carved away.