Undoubtely the biggest crime in watchtower world is apostacy. The upcoming DC apparently has quite some parts on this issue.
Recently I read a book from the philosopher Hubert Schleichert, the title is something like "How to discuss with fundumantelist without loosing your mind".
He goes into fanatics of all kind, and shows, why apostates are so dangerous for any ideology, may it be religious or even political.
The reason: Any strong ideology is convinced of possesing the truth. This creates a problem, since apparantly not everybody accepts this belief or ideology. However, those outside (world, pagans etc) did not have the chance or did not want to have the chance to fully get acquanted with the "truth" of a certain ideology. So they can be viewed as ignorant, blind whatever, and it creates not a big threat for the ideology itself. However, when those, who at one time shared the belief, turn away, those who seen the "light" turn on purpose to darkness, they must be outright evil, haters of truth, "mentally dsiseased" etc.
In history, every fundumanetly belief or ideology acts like this (example heretics in catholic church, dissidents of marxist world views etc)
They reason, the reaction is soo strong, is in my opinion, because the existence of those turning away from an established truth would lead to a question, a true believer never should ask: "Is there maybe something wrong with my belief ? " The only reason why people turn away from such an obvious truth can only be out of wickedness and hatred for truth. I cannot be because maybe it is not the truth after all.
Apostates, heretics and the like actually create a real problem for every belief. I think this even is worse for witnesses, who pride themselves in accepting people only after a thorough study of the belief system. How can someone fall away after "making the truth their own"?
Therefore its quite to be expected that a belef system like JW will react soo strongly to apostaes, its a defense mechanism, since its very foundation is endangered.