To preface, I would just like to say that I am only playing Devil’s Advocate by asking this question. This is by no means a "pleasant" question, but is nevertheless one which has been on my mind for a long time. In the beginning of Sefer Yehoshua, B’nei Yisrael enter Eretz Yisrael, and proceed to attack, destroy, and annihilate various cities throughout the region. In most cases, they attack without any semblance of provocation from the inhabitants of these cities. The reason that B’nei Yisrael do this, clearly, is that they were obeying the word of Hashem, who commanded them to attack these cities and conquere the land. Therefore, by doing so, they acted righteously, since they obeyed the word of Hashem, which, by definition, is righteous. I understand that no matter what action Hashem commands us to do, no matter how obsurd it may seem in our minds, must be carried out to the fullest. My problem is this; is there really any difference between the actions of B’nei Yisrael in Biblical times, who killed in the name of G-d, and the modern day Palesinians/Arabs, who kill in the name of G-d. Never mind the fact that the word of G-d recieved by B’nei Yisrael was true, and the word of G-d recieved by the Palestinians is not. The bottom line is, B’nei Yisrael acted as they did, killing thousands, simply because they believed that G-d commanded them to do so. Ditto with modern day Arabs. If an Arab truly believes that G-d wants him to be a terrorist, is there really and moral/ethical difference between his "hideous" actions today, and the "holy" actions of Bnei Yisrael many years ago.
Indeed, there is no difference other than the difference you pointed out, between truth and falsehood!!! This devil’s advocacy of yours is indeed the work of the devil, but not just by you. There is a general misuse of the intellectual faculty of man in which he compares things superficially, finding external similarities, and overlooking the inner, essential difference. For example, if one wishes to compare an animal to a man, and finds that the monkey or dog has 40 characteristics similar to those of a human, and JUST one difference – that this is an animal and that is a human. The vast majority of things are similar – 40 to 1!! But that one difference makes all the difference (see chapter 1 of Rambam’s "Eight Chapters – Introduction To Pirkei Avot"). Or let us imagine witnessing two people with knives cutting the flesh of another person. One is a surgeon and one is a murderer. But playing the devil’s advocate I could ask: what is the difference - both are cutting human flesh? If you say, yes, but one is healing and prolonging life, and the other is putting an end to life, that is true, but what’s the difference, the bottom line is that both are stabbing someone. And if the murderer also says that he is doing so for the good of mankind, does that make the acts equal? Is there really any moral/ethical difference between his "hideous" actions, and the "holy" actions of the doctor? A Jew that rises to kill a Nazi who is killing hundreds of people – are they both hideous actions? Both are killers!? Nazi propaganda also said that they were “doing G-d’s will” to rid the world of evil, parasitic creatures. If the evil says he is doing his evil for G-d’s sake or any other explanation, does that make it good and comparable to others who also say they are doing good and indeed are doing good? Throughout history, many have claimed to kill for the sake of good, but that certainly doesn’t make it good. And on the other side of the coin, does that necessarily mean that whoever claims to be killing for the good of the world is doing evil? The Source of Good and absolute morality tells us what is good and what is not; what has to be developed and what should be destroyed in order to purify the air of the world. As the wisest of all men said (Kohelet 3): “A time to kill… a time for war, a time for peace.” Indeed, when left to the scrutiny of man this can be a dangerous thing. The matter depends on the purity of the soul from which these actions stem. There are indeed bloodthirsty people who conduct wars to satiate their inclination to conquer and lust for power. And there is a nation whose only inner desire to live is to bring good to all. Even when G-d tells us that certain forces in mankind are an obstacle to the fulfillment of human perfection, the Jewish People find it difficult to do what must be done. Throughout history (and to this very day) the Jews received no satisfaction or pleasure from killing. For us, it requires overcoming our nature. For others, they must overcome not to kill – “by the sword you shall live.” (See Rav Tau’s "L’Emunat Iteinu", Vol. IV.) Yes, there really is such a thing as a war to end all wars, a war that comes to eradicate evil. And even if others also made that claim, does that mean that there cannot be one that really is the war to end all wars? Because there are imposters, does that mean there is no real McCoy? (Besides, your question seems inappropriate [or stemming from a double standard] coming at this time, when America went all out to kill and destroy in order to “uproot the forces of evil” in Iraq and “save the planet” from their weapons of mass destruction. Whether or not that was the true and only intention, the UN and most of the world went along with them.) Actually, your question is not new. Rav Kook has a letter answering this question over 93 years ago (Ma’amrei HaRiyah, p. 508). G-d’s wisdom that created the world and desires only the elevation of all Creation to the absolute Goodness that He knows (not the limited, subjective view of man that one generation thinks this is good and another time thinks another thing is best for man) revealed that certain elements withhold that Good for all. What cruelty to mankind it would have been had we left these corrupt, perverted cultures, even if due to what we deemed to be “moral considerations”? Rav Kook points out what the world has suffered (burning of children to the gods and other abominations of the Emori) because we did not fully abolish those negative forces (see Joshua 13:13; 16:10). What devastating darkness and evil would have covered the world and become a part of our culture and spiritual makeup had we been “merciful” and left these nations.