Obsessive devotion combined with thoroughly invasive attempts to convert people to whatever is being devoted to.
Devotion alone is not harmful. Teaching people about one's devotions or beliefs isn't harmful.
But people are completely out-of-line when they force people into unwilling bondage with the threat of death.
Usually, fanaticism is driven by power-hunger, corruption given by power, the strong desire to be considered right, misinterpretation of religious texts or other manifestos and documents, and/or insanity.
It's not often monetarily driven. That problem is usually called greed, an obsession with money. (Which I suppose might be a type of fanaticism.)
It is not belief, but fanaticism, that is the cause of many of the world's problems.
Some Christians either have a rather blasé attitude towards their faith, or they practise raving fanaticism which would be dangerous to the public at large if carried out in office. (Many Protestant churches are possesed by fanaticism.) These two stereotypes give Christianity a bad, bad, bad image that it's rather undeserving of, as most Christians are moderates, or are the blasé kind aforementioned.
Similarly, the Crusades and the Inquisitions were rather embarrassing fanatical events in history that have nothing to do with the message of Christ, but with the craze of power and being right.
Communism during the Cold War was sometimes fanaticism rooted in a state cult (like Stalinism), or in atheism. This fanaticism led to the deaths of many religious people, guilty of little to nothing else.
The fanaticism of the Ku Klux Klan led to the unwarranted deaths of many African Americans.
Nazi fanaticism killed Jews, communists, homosexuals, Catholics, and soldiers who could all have been doing something more useful than dying.
So, therefore, I believe strongly that it's not Christianity, Islam, Republicanism, Democratism, liberality, conservatism, Zionism, Judaism, capitalism, communism, socialism, dictatorship, or any other belief or circumstance which causes trouble. It is when these things hold too much influence over our lives and cause us to do horrible things. Or, fanaticism.