I think you can draw a line that 95% of humanity will agree on, like, killing children just cause they talk back to you. Or, you could define certain terms.
Like, a goal for ethical living would be to not increase suffering. It's still hazy, but that's your foundation, and you could add your various virtues, like working to alleviate suffering, and how well you're aware of the suffering you do cause. Like, it's not just that you don't knowingly cause suffering, but that you aren't willfully ignorant of it.
(And I'm not judging the person here, each person's individual acts are at issue)
That sets immorality as just knowingly causing more suffering in the world. And obviously that can be just hurting someone's feelings on purpose, to genocide. You don't get measured, but some things are still worse than others.