The Holy verse says:
"Lo! Allah hath bought from the believers their lives and their wealth because the Garden will be theirs: they shall fight in the way of Allah and shall slay and be slain. It is a promise which is binding on Him in the Torah and the Gospel and the Qur'an. Who fulfilleth His covenant better than Allah? Rejoice then in your bargain that ye have made, for that is the supreme triumph."
Lets first put the verse in its right context
The verses before the above verse was narrating the story of the hypocrites who abandoned the army in the battle of "Tabuk" and tried to spread rumors and discourage Muslims from fighting and finally they built a mosque known in Islam as the "mosque of Harm" or "Masjid Al-Dirar". This mosque was merely a cover to hide their activities against Islam. In the verse (9:107) GOD exposes their malicious plan and orders Prophet Muhammad, peace and blessings be upon him, NEVER to pray in this mosque. And so the verses before exposed the hypocrites and their evil deeds.
Then GOD gives Muslims the other image, the image of the true believers why? To ensure Muslims that they should not worry that these people abandoned them as long as HE HIMSELF is supporting them.
An insight in the Linguistics of the verses
1- Notice the use of the sentence (they shall fight in the way of Allah)and take very good care of the word (fight) which means an action that must have two sides. GOD did not say those who (they Kill) HE said (fight) which means they are facing an equally strong enemy. This is emphasized by the sentence right after (and shall slay and be slain) which means it was a fight between equals (two armies fighting) NOT as some ignorant people (or playing ignorance) are trying desperately to claim that this is an open call to kill civilians and that the Holy Quran is pushing Muslims towards killing blindly.
2- Notice the sentence (It is a promise which is binding on Him in the Torah and the Gospel and the Qur'an.) Which means the promise of paradise if you fight to defend your faith exists Not only in the Quran, but also in the Bible and the Torah. Always remember here that fighting in Islam is for self defense and to protect the freedom of choice of people and we have given several examples from the history of Islam that proves that Muslims NEVER and should NEVER oblige people to become Muslims by force. Remember the "Golden era of Jewish culture" and remember that this was when Spain was under the Islamic era.
3- The verse following the above verse says: " [Such believers are] the repentant, the worshippers, the praisers [of Allah], the travelers [for His cause], those who bow and prostrate [in prayer], those who enjoin what is right and forbid what is wrong, and those who observe the limits [set by] Allah. And give good tidings to the believers." (9:112) which continues to explain the characteristics of the true believers which means that a true believer is not just the one who fights (for the reasons that Islam dictated to fight and following the rules and regulations of Islam), but also the one who repents his mistakes, the worshipper, those who bow to God, those who encourage people to do right and discourage them from doing wrong and those who follow the rulings of God.
So Islam is NOT just about war and killing, it has a spiritual part like worshipping GOD, repenting your wrong deeds..etc. as well as real-life, social part like encouraging people to do right and social solidarity which is clear in making charity obligatory on capable Muslims (0.025% per year)...etc.
Trying to look at Islam as nothing but war is an unforgivable dishonest prejudice because this means deliberately overlooking the grand majority of Islamic jurisprudence and the Holy wisdom on which it is based which is actually an unacceptable offence.