Even in the OT there were rules for different situations, concerning if one was a citizen, or one was a foreignor, or one was an enemy, or criminal, or foreign government waging war against you. Even in the NT such distinctions are made.
2 Peter 2:12 - But these, as natural brute beasts, made to be taken and destroyed, speak evil of the things that they understand not; and shall utterly perish in their own corruption;
Jude 1:10 - But these speak evil of those things which they know not: but what they know naturally, as brute beasts, in those things they corrupt themselves.
You have to determine who is your "brother" and who is your "neighbor" and make differentiation between those who are your fellow citizens, and those who are an enemy bent upon your destruction. Even Jesus disciples had swords, small perhaps, for personal protection. The Apostle Paul traveled with soldiers for the protection they afforded against the criminal element that often attacked travelers.
Jesus had no problems with soldiers if they performed their tasks in a proper manner.
Luke 3:14 - And the soldiers likewise demanded of him, saying, And what shall we do? And he said unto them, Do violence to no man, neither accuse any falsely; and be content with your wages.
In Romans chapter 13 it's obvious there is a distinction between those who do good and the fate of those who do evil, and what the Islamic fanatics have been up to is definitely "evil".
3 For rulers are not a terror to good works, but to the evil. Wilt thou then not be afraid of the power? do that which is good, and thou shalt have praise of the same: 4 For he is the minister of God to thee for good. But if thou do that which is evil, be afraid; for he beareth not the sword in vain: for he is the minister of God, a revenger to execute wrath upon him that doeth evil. 5 Wherefore ye must needs be subject, not only for wrath, but also for conscience sake. 6 For for this cause pay ye tribute also: for they are God's ministers, attending continually upon this very thing. 7 Render therefore to all their dues: tribute to whom tribute is due; custom to whom custom; fear to whom fear; honour to whom honour. 8 Owe no man any thing, but to love one another: for he that loveth another hath fulfilled the law. 9 For this, Thou shalt not commit adultery, Thou shalt not kill, Thou shalt not steal, Thou shalt not bear false witness, Thou shalt not covet; and if there be any other commandment, it is briefly comprehended in this saying, namely, Thou shalt love thy neighbour as thyself. 10 Love worketh no ill to his neighbour: therefore love is the fulfilling of the law.
You CAN have love toward your fellow man, but it is tempered with the realization that one must also be ready to deal with evil when it seeks to destroy you, your loved ones, your people, your nation. Such "brute beasts" are not our "neighbors" and deserve the sword.
Romans 12:18 - If it be possible, as much as lieth in you, live peaceably with all men.
Unfortunately it's not always possible to live in peace with others, certainly not when your destruction is their aim. Almost all the commandments and laws concerning "love thy neighbor" are concerned with how citizens and foreignors that live within one's country deal with each other. Most of the things taught in the NT were the same as the OT on how to deal with one's "neighbour", there was a different set of rules on how to deal with foreign countries, those with whom you were at war with. There were rules on proper treatment of POW's too. Unity and love for one's fellow citizens has nothing to do with defending one's self against enemies from outside, nor criminals from within a country. Some interesting word searches on certain words below, the number of times certain words are used in both Old and New Testaments.
"love" was found 645 times in 546 verses.
"hate" was found 190 times in 179 verses.
"enemy" was found 110 times in 109 verses.
"friend" was found 107 times in 99 verses.