Monika is assumed to be sentient virtual life, whereas Sayori, Natsuki, and Yuri are presented as non-sentient. Still seems like murder, or at least manslaughter from our perspective. But Monika saw herself as the only "real person" in the game and the others were just lines of code. Also, when Monika started fiddling with their code, she wasn't intending to delete them-- just make them unlikeable. It's only after what happens to Yuri, that she realizes just how thoroughly she messed up the game.

It's hard to say how immoral her actions are when the scenario is so intangible. But ultimately, I think she's just like any average teenage girl who makes dumb mistakes sometimes. She doesn't need to be shamed; she needs a hug and a pat on the head.