It's not unusual to feel guilt in your situation, but it is misguided. Your parents are trying to hold you responsible for something you have no control over: your beliefs. No one can truly decide to believe anything (though they can decide to PRETEND to believe things they really don't) because beliefs are simply conclusions that our minds draw from the available evidence. You can no more choose to believe in the Watchtower BS (Belief System) than you could choose to believe that the Earth is flat, motionless and lies at the center of the cosmos. Once you understand something, you can't ever go back to not understanding it.
That said, your parents probably should feel guilty. While no one can control their beliefs, most people can control their behavior to some degree. If they are going to shun you simply because your mind isn't capable of ignoring the overwhelming evidence against the Watchtower BS, then this is a behavior they have definitely decided upon and they are totally free to change it despite the threat of emotional blackmail they are living under.