Does Islam support killing of kafir (unbeliever)?

Many scholars opine that to kill a person there are very limited reasons in Islam. That's why they claim it to be the religion of peace. But some people point out that it is mentioned in Islam to kill *Kafirs* too. The word "Kafir" I am referring to means someone who does not believe in one God. I am new to this topic. I have heard this from many people and think this is right place to ask.