Every time when I see keyboard warrior talk about karma. I feel that a lot of them often doesn't realise that their actions are not exclude from the karma law.
When we are insulting / scolding / humiliating peoples online, we are actually creating bad karma for ourself. The whole point of understanding karma is not for us to justify our bad action, like getting our revenge by doing bad things towards other people.
Let it serve as a reminder for ourself. Are we doing good deeds or are we doing bad stuff? Understand how the choice we made will negatively or positively affect our life.