According to Watchtower, 'THE BIBLE' says that JW's who leave the Watchtower Society (i.e. apostates) are 'mentally diseased', especially if they disagree with the organization's teachings.
'Paul described such individuals as being “puffed up with pride, not understanding anything, but being mentally diseased over questionings and debates about words.” (1 Tim. 6:3, 4) Could Timothy risk toying with the harmful ideas that were infiltrating the congregation? No, for Paul urged Timothy to “fight the fine fight of the faith” and turn away “from the empty speeches that violate what is holy and from the contradictions of the falsely called ‘knowledge.’” (1 Tim. 6:12, 20, 21) ... Interestingly, Timothy was told that in “later periods of time some [would] fall away from the faith, paying attention to misleading inspired utterances and teachings of demons.” (1 Tim. 4:1) All in the congregation, including those in positions of responsibility, need to be like Timothy in displaying strong, unwavering faith. By taking a strong stand and decisive action against apostasy, we can make our advancement manifest and be an example in faith.' 
'The Bible says that apostates are mentally diseased. ... We do not speak to them ... We also do not read their books, watch them on television, read what they write on the Internet, or add our own comments about what they write on the Internet. ... We must always be careful not to think like people in Satan’s world. ... remember that Satan wants to fool us, and no one knows better how to fool people than he does.' 
1. Watchtower Magazine, Dec 15, 2009, Article: 'Make Your Advancement Manifest', par 16, 17
2. Watchtower Magazine, July 15, 2011, Article: "Will You Pay Attention to Jehovah's Clear Warnings?", p 11-16; par 5-13
2b. The above 2011 Watchtower quote came from a long article warning JW's about Satan's world outside the Watchtower organization.
2c. The information was actually 'studied' in depth by all JW's at their public congregation meetings and in private.
2d. At the bottom of each page the Watchtower asks questions. JW's must answer 'correctly' based on what the Watchtower teaches.
3b. Interestingly, the Watchtower on rare occasions has admitted they are not inspired by God or His angels. They say mistakes are possible. Thus some or all of what they teach could be incorrect. Read Watchtower quotes about having no divine inspiration
4. Read more Watchtower quotes about mentally diseased JW's. Apostate Jehovah's Witnesses