Every person on the earth, who is not a Muslim is a KAFIR, it is not true.
Meaning of KAFIR is considered wrongly by many people, let's first see it's meaning
disbeliever/unbeliever, faithless, apostate, atheist etc.
A person who don't know anything about God or Quran, he is not a KAFIR but those after knowing the truth, reject to accept it those are KAFIR.
Uma, already gave enough proof that a person should not be killed but he would have to give the Jizia (tax) and he can live with them.
Quran 8:55 Indeed, the worst of living creatures in the sight of Allah
are those who have disbelieved, and they will not [ever] believe