>>81473637In the best scenario someone another person has grown close with has suddenly vanished from their life, and this alone can be a very painful experience depending on the level of closeness and their personal situation. If we consider the example I gave about posters here, most people here have had problems with social stigmatization or loss so they are far more susceptible to social pain. So it stops being just insensitive and enters the realm of cruel and abusive.
Now imagine the worst scenario, an avoidant with someone in a serious romantic relationship. Serious relationships have very intimate feelings, and are at the stage life plans riding on a shared future get involved. They potentially even have expenditures, assets, or kids. Someone walking out on that because they became afraid would be completely destabilizing to another person's life and cause significant harm to them. This situation is not likely to be found here, but it shows how extreme this behavior can become if it is unchecked.
Insensitive is shouting at someone in frustration, you are massively downplaying the potential levels of harm that this behavior can reach depending on the scenarios and levels of closeness at play. Using such a mild word implies the person with these issues doesn't need to seek help for them either. Because of it's potential for significant harm and because not seeking help for this means someone is choosing to mess up people they care for, evil is apt.