if everything they teach is scriptural why would they leave?
This is a good question. As many have demonstrated on various threads here, and in articles at other sites, not everything the Watchtower Society teaches is scriptural. In fact, I have found very little that is. They have only made their teachings appear to be scriptural by taking scriptures out of context (the same way they often take quotes from secular sources out of context) and bending and shaping and twisting them to fit into their own design.
Having left JWs and having left Christianity entirely for over a decade, then having recently taken up reading the Bible on its own and reading the JW publications, the differences I find between the two are astounding, even moreso than I had noticed while growing up with JW teachings.
When reading their publications, try reading the greater context of the scriptures they cite. Ponder it. Don't jump from place to place. Question everything you read, including what is in the Bible. Don't let your questions be too easily ignored or set aside. If you don't have questions yet, consider other people's questions. Check out some Bible commentaries. (Try some of the free resources at e-sword: http://www.e-sword.net/commentaries.html )
And be wary of those who put more emphasis on what to think rather than how to think.
Best wishes!
~Merry