Don t judge Islam by its followers. If you want to judge how good Islam is then judge it according to its authentic sources, i.e. the Glorious Qur an and the Sahih Hadith. If you practically want to check how good a car is put an expert driver behind the steering wheel. Besides Muslims, there are several honest and unbiased non-Muslim historians who have acclaimed that prophet Muhammad was the best human being. Kafir means one who rejects.
Kafir is derived from the word kufr, which means to conceal or to reject.
In Islamic terminology, Kafir means one who conceals or rejects the truth of Islam and a person who rejects Islam is in English called a non-Muslim.