Fighting and Killing is an Islamic Principle.

The God of Islam in The KURAN (Quran) says:
Then Fight and slay the Pagans (every non-muslims) * wherever ye find them, and Seize them (altawba 9:5)
Fight against those who (1) believe not in Allâh, (2) nor in the Last Day, (3) nor forbid that which has been forbidden by Allâh and His Messenger (4) and those who acknowledge not the religion of truth (i.e. Islâm) among the people of the Scripture (Jews and Christians), until they pay the Jizyah with willing submission, and feel themselves subdued. (altawba 9: 29)
Allah told them to kill non Muslims any where and they will get many Virgins Girls (from 41 to more than 12 500 Virgins Girls for each Muslim man ) in their Paradise as a Reward for obeying Allah and killing the non Muslims people in there Holy War: i.e. THE JIHAD
Look to some Kuran Teachings that Teach Musilms
To Fight and Slay the Non-Muslims
The Way of those on whom You have bestowed Your Grace, not (the way) of those who earned Your Anger (such as the Jews), nor of those who went astray (such as the Christians). (alfatiha 1:7)
Fight them, and Allah will punish them by your hands, cover them with shame, help you (to victory) over them, heal the breasts of Believers, (altawba 9: 14)
And yet another translation
Fight against them so that Allâh will punish them by your hands and disgrace them and give you victory over them and heal the breasts of a believing people, (9:14)
But when the forbidden months are past, then fight and slay the Pagans * wherever ye find them, and seize them, beleaguer them, and lie in wait for them in every stratagem (of war); (altawba 9:19)
And yet another translation
Then when the Sacred Months have passed, then kill the Mushrikûn wherever you find them, and capture them and besiege them, and prepare for them each and every ambush. But if they repent and perform As-Salât (Iqâmat-as-Salât), and give Zakât, then leave their way free. Verily, Allâh is Oft-Forgiving, Most Merciful. (see V.2:105) (altawba 9:19)
(V.2:105) Neither those who disbelieve among the people of the Scripture (Jews and Christians) nor Al-Mushrikûn (the disbelievers in the Oneness of Allâh, idolaters, polytheists, pagans, Christians how worship Jesus as God, etc.) like that there should be sent down unto you any good from your Lord. But Allâh chooses for His Mercy whom He wills. And Allâh is the Owner of Great Bounty. (albaqara 2:105)
And in the following verses you will see that Muhamad identified Christians as Disbelievers and Mushrikûn because he did not understand the Christian faith neither the Trinity - the Godhead in the Bible - nor the personality of Jesus Christ as the Eternal Son of God. Therefore, Muhamad considered Christians as Mushrikûn.
According to him, disbelievers are those who say:
"Allâh is the third of the three (in a Trinity)." But there is no ilâh (god) (none who has the right to be worshipped) but One Ilâh (God -Allâh). And if they cease not from what tthey say, verily, a painful torment will befall the disbelievers among them. (almaida 5:73)
They indeed have disbelieved who say: Lo! Allah (God) is the Messiah (Jesus Christ ) , son of Mary (almaida 5:17)
Fight against those who (1) believe not in Allâh, (2) nor in the Last Day, (3) nor forbid that which has been forbidden by Allâh and His Messenger (4) and those who acknowledge not the religion of truth (i.e. Islâm) among the people of the Scripture (Jews and Christians), until they pay the Jizyah with willing submission, and feel themselves subdued. (altawba 9: 29)
The punishment of those who wage war against Allah and His Messenger, and strive with might and main for mischief through the land is: execution, or crucifixion, or the cutting off of hands and feet from opposite sides, or exile from the land: that is their disgrace in this world, and a heavy punishment is theirs in the Hereafter;(almaida 5:33)
Fighting is prescribed upon you, and ye dislike it. But it is possible that ye dislike a thing which is good for you, and that ye love a thing which is bad for you. But Allah knoweth, and ye know not. (albaqara 2:216)
O Prophet! rouse the Believers to the fight. If there are twenty amongst you, patient and persevering, they will vanquish two hundred: if a hundred, they will vanquish a thousand of the Unbelievers: for these are a people without understanding. (alanfal 8:65)
Based on the previous verses, we conclude that Islam considers Christians as Pagans, Disbelievers and Mushrikûn, because they:
1- Believe not in Allâh of the KURAN, nor forbid what has been forbidden by Allâh of the KURAN and his Messenger.
2 - Do not accept Muhamad as Prophet and Messenger from the True Allâh (LORD God )
3 - Acknowledge not the religion of Islam as it is From God.
4 - Worship Jesus Christ as the Eternal Word of God.
5 - Believe that the true Allâh is the One True God In His Trinity as revealed in the Holy Bible.
Thus, Islam teaches that Christians and others should be Killed in the name of Allâh.
Islam pushes Christians to believe that Jesus Christ is not more than a prophet otherwise they will be KILLED by muslims.
Again
Fight against those who (1) believe not in Allâh, (2) nor in the Last Day, (3) nor forbid that which has been forbidden by Allâh and His Messenger (4) and those who acknowledge not the religion of truth (i.e. Islâm) among the people of the Scripture (Jews and Christians), until they pay the Jizyah with willing submission, and feel themselves subdued. (altawba 9: 29)