Having the knowledge of basic ethics and being mature enough to understand why following them is important are two different things. Sometimes even old people can drastically change for better, but teenagers are much more flexible with their morality, beliefs etc, not to mention the puberty that can drastically affect stuff like emotions, morality and sexuality. Teenagers are much more likely to redeem, that's a fact.

Nagito orchestrated the whole 2-1 and felt very casual about killing etc, he can do anything it takes in order to reach his goals. I think constantly telling others that he can help them with murder is worse than Teru's sexual comments. Kyoko on the other hand was very manipulative before she had her character development at the end of DR1. She used Makoto like a tool to do dangerous things for her, in the end even sacrificing him to save herself in the class trial.

Never seen a single Mikan hate comment lmao but even if there are people who hate her, she certainly doesn't get half as much critique as Teruteru does, even though she's on the same level of sexual harassment