Well, I might believe in God so much that I might accept any instruction from a religious authority
Those people are not God, and once again, I reference Jeremiah 14:14b. If you read into it, Radical Islam is just that, completely radical. fewer than one percent are actually willing to do that.
Actually, there is not radical/fundamental Islam, there's just radical people who choose to follow Islam as it is interpreted by the radical authorities, and then there are non-radical people who don't follow Islam's all the bloodiest orders but still call themselves moslems, but that's neither here nor there. Suffice to say that the basic tenets of Islam actually do aim to completely Islamic world with no other religions, and that's the basic line where that religion stands.
And I would also like to raise the question, if you know that priests, clergymen and whatnot are not God, why then Bible is? Did you know personally the people that wrote the Bible and formed the canon of Bible as we know it? I certainly didn't. And even if I had known them I still couldn't be sure if the text came from them or God.
You're still stuck in the loop of circular reasoning. There is no way we can know whether or not the Bible actually was written under divine influence.
There are plenty of examples where faith in God has led to rather grievous consequences.
Most of which happen at the hands of the criminal, the psychotic, and the radical.
And how is their faith different from yours?
Or is it so that true faith only causes positive things, and thus bad things seemingly caused by faith are actualle results of false faith?
If that is what you think, it stinks.
(after all they do have God to guide them so obviously they are right, right?
According to the Bible, so do you. That said, you are not always right. people are by their nature imperfect and subject to temptations (even Jesus was tempted) If you assume that any priest, pastor, etc. has the divine right of God to tell you to do somethingm suicidal or murderous, you are being a stupid misguided idiot. 
Well right back at you, if you assume that the authors of Bible, every prophet and historian and rabbi and apostle in Bible, and the Council of Nicaea, and all the theologists that have been deciding what the Bible means had the divine right of God to tell you to do or not do something, you are being stupid misguided idiot.
I'm certain you can see where this is going. The basic assumption that the Bible is correct is based on the Bible itself. There's no source criticism here at all, so let's practice it a bit:
Assuming that the Bible is from God does not make it certain. Quite like reading something in the Internet doesn't make it true. Or like Siddhartha Gautama said, don't believe in old stories just because they are written down. The whole thing with religions is built on cascading evidence, and if the basic assumption is abandoned, the whole thing loses a lot of it's certainty. After questioning the origins of religions, taking things like "you go to hell if you don't believe in God" at face value seems rather silly, and instead one starts to think
why would I go to hell if I didn't believe in God. And after questioning the divine truth of the Bible, the traditional explanations of Christian theology - that man is inherently flawed and needs the faith to be salvaged from his sins - become compromised and unconvincing.
the Westboro Baptist Church believe that everyone that is not part of the Westboro Baptist Church is going to hell.
That would fall under the heading of 'radical'
Well they believe so, why is their belief wrong?
The Bible is quite firm on the concept of Hell. My own view of Hell is eternal separation from Him. Others may prefer to see it as a fire and brimstone scenario. However, my point stands that the 'not believeing in God' section would have serious consequences (read: Hell).
The Bible can tell me many things but it can't prove itself to be true. The question is,
why would not believing in God have serious consequences. Try to explain it without using the Bible as a source. Why would God put so much importance on faith that it alone would decide your fate in the assumed afterlife?