Having had a moment to look at the context of the texts in question, I think there are rich pickings in explaining the arrogance of the Watchtower on the matter of calling ex- believers “mentally diseased”.

The point being that the Biblical text itself is quite perverse. It is illogical and exhibits desperation. Its writer Paul had two salient facets to his character; he was both well educated but was also given to religious delusions (“Whether in the body or not...” 1 Cor 12:1-4). He indeed spearheaded his brand of the Christ-cult, one of many such cults based on the archetypal Christ figure; a wonder- worker, healer, teacher and saviour back in the first century. His objective as shown in the letter to Timothy was to protect his own fledgling cult, which was yet to be associated with the name Jesus. How did he do this? 

The text says he told Timothy (1Tim 6:1,2) that slaves should remain bonded (in bondage) to their masters even if the masters are believers. Fair enough, if that’s what you want, but he follows this edict with what amounts to a curse, something quite familiar in contemporary literature of the time. He made the ruling (on slavery) and then roundly condemns those who might choose to disagree. Paul has a hissy fit (verse 3) “If anyone teaches differently” he spits and thunders, “then he is a conceited idiot” (Phillips translation)”His mind is a morbid jumble of disputation and argument” (literally : sick about questions)and from here on, he claims, it follows that it will all go pear shaped. Verse 5 speaks of being corrupted in mind (mentally diseased?) and the whole episode then calms down with admonition to just be content with what we have. It is an emotive outpouring, a blunderbuss like threat, reminiscent of the rhetoric of infamous dictators of the twentieth century.

Really, if someone disagrees with you; does that make them an idiot? Is that a reasonable consequence? Is this divine reasoning?  Paul is surely not just intemperate but also an insensitive bigot. Another of his gems is his endorsement of the statement “All Cretans are liars, evil beasts and lazy gluttons”. (Tit 1:12)

“Mentally diseased” is not such a bad translation in fact. I think the KJV and the new NWT rendering is actually underplaying the force of the original. Nevertheless we have here one of the many examples of irrationality, it is self-interested, it is a sectarian name-calling which the Bible not only contains but happily promotes. It is therefore hardly surprising that the Watchtower, whose life-blood is teaching and defending the Bible as the sacred word of God, deploys this Bible-based bigotry in their attempt to mind control.