It isn't even really overconfidence. We don't have to know the koran better than those people in order to reject it. If I can compare three different translations of koranic verses into english to each other, I feel I have a reasonable degree of accuracy as to what the verse says, and from there I can judge its contents. What we don't have is your predetermined conclusion that this book is somehow divinely inspired wisdom, or in any way a good guide on how to lead a good life. It contains exactly what I would expect from a pseudohistorical collection of iron age folklore.
You, however, have to go through a series of rather impressive mental gymnastics wherein you don't read the text at face value, but rather think up things it could say instead and then leap immediately to that definitely being the most likely meaning above all others. It's a practice common in religious people, and really it's a form of denial. Christians do the same thing, especially with uncomfortable things in the old testament.
As for the finger-chopping, I think the verse was Koran 8:12
Also, being an american of turkish descent, am I correct in assuming that you also don't speak arabic?
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I'm bored out of my skull, let's play a different game. Let's pay a visit down below and cast the world in flame.