The first step in learning about Islam is to know the right words. The language of Islam is dualistic. There is a division of humanity into believer and kafir (unbeliever). Humanity is divided into those who believe Mohammed is the prophet of Allah and those who do not.
Kafir is the actual word the Koran uses for non-Muslims. It is usually translated as unbeliever or infidel, but that translation is wrong. The word unbeliever is neutral, while the attitude of the Koran towards unbelievers is very negative. The Koran defines the Kafir as hated by Allah. A Muslim is never the true friend of a Kafir. Kafirs can be enslaved, raped, beheaded, plotted against, terrorized, and humiliated. A Kafir is not a full human.
When you read the complete Islamic doctrine of Koran, Sira (the biography of Mohammed), and the Hadith (the traditions of Mohammed), you will find that Islam is fixated on the Kafir. Over half of the Koran is about the Kafir, not Muslims. It is the stated purpose of the Islamic textual doctrine to annihilate every Kafir by conversion, subjugation or death. Jihad can be waged against the Kafir.
Kafir is the worst word in any language. Other words can be hurtful, but those words are personal and are a personal opinion. But, Kafir is the word used by Allah. So the hatred in the word Kafir is sacred, the hate of Allah.