It is true Christians have done terrible things throughout history. But they've always been acting in opposition to their Christian faith. There are no New Testament verses that say, Kill unbelievers.
Abel's sacrifice was accepted by God because it was made according to God's instructions. Cain, however, tried to approach God in his own way and was rejected by God (Genesis 4:3-5). From that point on, Cain became a symbol of those who try to approach God on their own terms.
Unlike many people today, Jesus did not teach that all religions in the world are simply different paths up the same mountain of truth that leads to God. Jesus could not have been more clear: he offers the only way to heaven.
When commanded by the Jewish officials to quit invoking the name of Jesus Christ, Peter responded, "And there is salvation in no one else: for there is no other name under heaven that has been given among men, by which we must be saved" (Acts 4:12). Remember, Peter was a Jew, speaking to a group of Jews, claiming that there is no salvation for Jewish people - or anyone else - apart from faith in Jesus Christ.
If trusting in Christ is the only way to be saved one has to wonder about infants, small children, and those who are child-like who are incapable of exercising faith in Christ.