The reality is the other person may not even be aware that they have caused such ill will. What you find as utterly reprehensible behaviour, might not even register with them. Good manners and proper etiquette, while important for everyone, could possibly get overlooked. Not because of a lack of significance, it could simply be ignorance. Everyone does not know your rules and sometimes it is our perceptions that are out of whack.
I believe this is behind much of the cyber bullying that goes on. It is very easy to reach out to friend or stranger and expect an answer only to be blanked. It may cause hurt feelings that will lead to the imagination running wild with all sorts of mistaken outrage. And it all boils down to missed messages, deleted notifications or a bad Internet connection.
The same can be said for real life. Although the Internet cannot take the blame. Sometimes the problem is a straightforward lapse in communication. Most people do not enjoy deliberately wounding another. Yet, rudeness abounds. It is far better to offer a bit of lenience. That is not to say, you should allow for another if inflict physical harm.
If you are feeling let down by the actions or non-actions of someone else, it would be magnanimous of you to submit, to yourself, that they are struggling and give them the benefit of doubt. To find that they mean you pain, will surely sting. It is better to walk away. Who needs people like that in your life?
For as long as people have been interacting, there have been differences of opinion. Yes, you are right. For you. With that being said, they are right. For them. There is no need for hard feelings. One can only control their own actions and reactions. Choose to cause no offence.