salam brother and sisters
br suhel what i said was from personal experience, I my self have been called a Kafir a a brother some time back and more recently another brother a revert to islam by the name of muhammad was called a Murtad also by a muslim, so brother i did not make it up.
You are right calling a msulism a kafir is very bad since one of the person either the accuser or the accused is a kafir as a hadith of the prophet (SAW) says.
The problemn here is that soem brothers can not take or conduct proper rational dialogues, they run on their emotional values and stick to what they thing is Absolutely correct and therefore everyone else is wrong.
What is lacking is an open mind and the lack of searching for the truth, often truth hurts butif its the truth then we should lean to take it no matter how hard it is.
Wassalam