Taken out of context, one would be tempted to interpret this in the way you describe. However, both the prophet and the Quran insisted on the importance of taking the book as a whole.
Anyway, what we're dealing with in this verse is brought to light in the preceding one.
"What aileth you that ye are become two parties regarding the hypocrites, when Allah cast them back (to disbelief) because of what they earned? Seek ye to guide him whom Allah hath sent astray? He whom Allah sendeth astray, for him thou (O Muhammad) canst not find a road." (Qur'an 4:88)
Ibn Katheer explains the verse as follows:
"And His [God's] saying "They but wish that ye should reject Faith, as they do, and thus be on the same footing (as they)" means that they (the hypocrites) wish you misguidance so as to be like them in it; and this is due only to their great animosity and hatred towards you, that's why God said "so take not friends from their ranks until they flee in the way of Allah(meaning left al-hijrah)" as said by Al-Awfy based on Ibn Abbas, and Al-Saddy said "they showed their kufr, then take them and kill them wherever you find them, "and take no friends or helpers from their ranks" means not to take any of them as allies or seek their support against the enemies of Allah as long as they are like this; then Allah excluded from them."
So, the hypocrites are the only ones whose killing is condoned. To understand who are those hypocrites, one needs to look at Sura 63 is called "Al Munafiqoon" (literally "The hypocrites"). A rough knowledge of the history of Hijra (migration to Medina) shows that during these days, a bunch of people were trying to bring down Islam from within.
Hence, the people who "turn their backs" in 4:89 are the Munafiqoon who were actively fighting Islam while pretending to profess the faith. I think we can all agree that it's nothing but self-defense.
As you can see, once put back into context, the meaning is entirely different. You may argue that in all his wisdom, God the omnipotent could have made the book fool-proof to avoid the abuses we witness (the Saudis, Talibans, Al-Qaeda...) from happening, but I think you're a believer yourself and will abstain from doing so.
Yes sir, I do know the Quran.
Sura 18 is Surat Al-Kahf (the cave) and narrates the story of the sleepers of Ephesus which I'm sure you're familiar with.
Anyway, the passages you quoted are pretty straight forward once you start the Sura at verse 60. I'll give you a quick and dirty summary but encourage you to read it by yourself (read the whole Sura; It reads very easily).
While out in the desert, Moses ran into a "slave" ('Abd) of Allah whom the Almighty bestowed a great deal of his knowledge upon (probably a metaphor to refer to some extraordinary being like an angel charged with a mission). Moses knew what that he was dealing was someone sent by God, so he asked to follow him in the hope that he could learn from him. The "man" warned Moses that he won't be able to tolerate what he will see and will not act impatient. Moses assured him that he intends on displaying the utmost patience.
After Moses agreed to keep his mouth shut and don't question anything the "man" would do. First, they stop by a boat and the "man" drills a hole in it. Moses, outraged, tells the "man" that he'll be causing the death of innocents. The "man" reminds Moses of his promise, and Moses immediately apologizes for his inappropriate behavior.
Then comes the killing of the little boy (Didn't mention how the killing was done - on highway '61). Moses predictably expresses his utmost discontent. He is then reminded of what they agreed upon, and Moses apologizes for forgetting yet again, and asks for one final chance. The strange duo comes then to village, ask to be fed, and the inhospitable inhabitants tells them to beat it.
On their way out, they noticed a structural crack in a wall that could have fallen out any moment. The "man" rushed to repair it and then resumes the journey. Moses, who forgot the pact yet again, mentioned that he could have demanded to be paid for the job. The "man" then explains how the boat belonged to poor hardworking fishermen and that a belligerent king (who am sure was waging "preemptive" wars) was confiscating every able boat.
Drilling a hole in the boat would actually turn out to save them and their families from starvation. The boy he killed was going to "yoorhikayooma" (which is a heck of lot more than a burden). As for the wall, it belonged to two orphan boys in the city. Under it, there was a treasure that belonged to them. Had the wall fallen and they found the treasure at such a young age, it would have been dilapidated (stolen, taken advantage of by a greedy newfound "guardian"...) in a speedy fashion. It was better to wait until they matured to reveal the treasure to them.
The message here is that God knows best and we shouldn't question his actions before everything ultimately has a purpose. Don't ask me what 'cause I don't know myself. Only God knows the future, intentions of people and all the other hidden details.
Can this (metaphorical? real?) story be abused. Sure! Anything that can be abused will be (ask Murphy). Once you think that you're the hand of God, the voice of God, talk to God bidirectionally or any of the other crap Al-Zawahiri (or Bush for that matter) believes, there is no limit to the damage you can do and rationalize. You'll always be able to get away with killing, maiming, stealing or drilling holes in boats.
Now, you still didn't answer my question from the previous post. Do you think that...
a) Islam is a barbaric religion which I have no clue about, and the only ones to have studied it, taken it seriously, and comprehended the message are the likes of Al-Qaeda.
b) I accepted the faith fully aware of the "barbaric nature" of it.
c) Islam is a message of peace which some politically motivated extremists, petty criminals, and mentally disturbed are twisting.
d) None of the above.
Which is it?
P.S: I believe Voltaire nicked a great deal of his work from this Sura.