First of all you should understand the motives for jihad in Islam, before asking about who deserves to be killed.
These motives were summed up by Rib’i ibn ‘Aamir and other companions of the Prophet (peace and blessings of Allaah be upon him) and were stated to Rustam, the commander of the Persian army, in the battle of al-Qaadisiyyah, when he asked them one after another, for three consecutive days before the battle took place: “What is this that you have brought?” The answer came: “Allaah has sent us to bring forth whomsoever He wills from the worship of people to the worship of Allaah alone, from the hardship of this world to its ease, from the injustice of other religions to the justice of Islam. He sent His Messenger with His religion to His creation. Whoever accepts it from us, we will accept it from him and will go back; we will leave him and his land alone. Whoever refuses, we will fight him until we end up in Paradise or in victory.”
This religion has always – and continues to and always will – be faced with resistance based on deviant beliefs and false ideologies, with political, military, social, economic and racial obstacles. All these obstacles combined may produce an attitude that rejects Islam altogether and prevents people from following it.
If verbal discussions and debates, arguing in a way that is better, address deviant beliefs and ideas, then the jihad movement addresses other, material obstacles, foremost among which is the political power that is based on those combined factors. It aims its strikes at the military and political powers that enslave the people to something other than Allaah – i.e., rules them by means of something other than the laws of Allaah – and that prevent them from listening to the truth and following it with absolute freedom. Thus there is a need for both da’wah and jihad in order to face the real obstacles of life.
See Ma’aalim fi’l-Tareeq, p. 59 ff
Thus it becomes clear that the basic principle of fighting as prescribed in sharee’ah is jihad, and its purpose is that all religion should be devoted to Allaah, and that the word of Allaah should be supreme. Whoever tries to prevent that is to be fought, according to the consensus of the Muslims. With regard to those who are not involved in resistance and fighting – such as women, children, monks, old men, the blind, the chronically ill, and so on – they are not to be killed, according to the majority of scholars, unless they fight in word or deed… because we are to fight those who fight us, if we want the religion of Allaah to prevail, as Allaah says (Interpretation of the meaning):
“And fight in the way of Allaah those who fight you, but transgress not the limits. Truly, Allaah likes not the transgressors”
[al-Baqarah 2:190]
It was proven that our Prophet (peace and blessings of Allaah be upon him) passed by a woman who had been killed in one of his battles, and the people were standing around her, and he said: “This one was not a fighter.” And he sent a man to Khaalid ibn al-Waleed – who was a general at the head of his army – and commanded him: “Tell him not to kill any children or hired workers.” Perhaps what is meant is those who were not bearing arms. Narrated by Abu Dawood, 2669; al-Albaani said: It is hasan saheeh; Saheeh Abi Dawood, 2324.
Allaah has permitted killing to whatever extent it is needed to bring people to the right path, as He says (interpretation of the meaning):
“and Al‑Fitnah (tumult and oppression) is worse than killing”
[al-Baqarah 2:217]
i.e., although killing involves evil and mischief, the evil and mischief caused by the fitnah of the disbelievers (the tumult and oppression that they cause) is worse than that. With regard to the person who does not try to prevent the Muslims from establishing the religion of Allaah, his disbelief harms no one but himself.
Al-Siyaasah al-Shar’iyyah by Ibn Taymiyah, 165-167.
Here there is an important point to be made: fighting, aggression and oppression are old problems that affect mankind so long as people are influenced by greed and by whims and desires, and so long as both good and evil exist among men. Ibn Khaldoon said:
“Wars and all kinds of fighting have existed from the moment Allaah created mankind. Their origin is the desire that some people have to wreak vengeance on others… This is something natural in man and no nation or era is free of this...”
Moreover, what do the Muslims have to do with the terrible wars that have taken place between various Christian groups in the past, and what befell the Protestants at the hands of the Catholics?
What do the Muslims have to do with the two World Wars, or with Hiroshima and Nagasaki?
We have the right to say, along with Count Henri de Christie: “It is more correct to say that the peaceful nature and gentleness of the Muslims is the reason why their state fell.”
Allaah the Almighty has spoke the truth:
“Those who have been expelled from their homes unjustly only because they said: ‘Our Lord is Allaah.’ For had it not been that Allaah checks one set of people by means of another, monasteries, churches, synagogues, and mosques, wherein the Name of Allaah is mentioned much would surely, have been pulled down. Verily, Allaah will help those who help His (Cause). Truly, Allaah is All‑Strong, All‑Mighty”
[al-Hajj 22:40]
Think about that, then listen to the words of the Prophet (peace and blessings of Allaah be upon him): “Allaah wonders at those who will enter Paradise in chains.” (Saheeh al-Bukhaari, 3030). And he said: “[The verse] ‘You are the best of peoples ever raised up for mankind’ [Aal ‘Imraan 3:110] means, You are the best for the people, as you bring them with chains on their necks till they embrace Islam” (Narrated by al-Bukhaari, 4557) – i.e., when they fight others, they may take some of them prisoner, or others may submit to their rule, then they come to know Islam and enter into the faith, even though they disliked it at first.
http://www.islamqa.com/en/ref/27180/
