Maru was originally designed as a girl so my headcanon is that girl Maru's story could have been very different from what we see in game. I have no reason to believe that other than disliking that part. Maru's story ties in two elements of the Shizue Fueti stories, death by urination and the involvement of insects, so it was probably never going to be different.

Maru's theme is self-disgust leading to repression of the memory of guilt, which compelled a lifelong pattern of trying to be the opposite of who he knew he really was in order to avoid remembering himself. I don't like how it was done.

So, fix-fanfic girl Maru would not have exercised cruelty to animals. Instead girl Maru started her school career as a tomboy and a bully who had one particular girl target. Girl Maru one day forced her to drink all the leftover milk which was intended for disposal, a significant amount of which was spoiling because she wanted that girl to piss herself in fear in order to see her power over her made visible. The bullied girl never pissed herself and Maru had to let her go at some point in order to go home. They never saw each other again but Maru learned she made the other girl very sick and her parents had to pull her from school. Maru was harshly beaten by her mother after that and her parents never saw her the same way again so she tries to avoid seeing her former self in the mirror. Maru thinks perhaps that Saeko might be the same girl she used to bully (denied in the following night conversation). Maru being terrified to the point of urinating in fear even though the girl she bullied never did. The conversation that follows is about misremembering yourself instead of about accurate memories.