If you don't know why Jehovah's Witnesses are a cult, the simple answer is because JW's deny the deity of Jesus Christ, deny the Trinity, and believe in a works-based salvation. You can click here to watch several videos from Living Waters: "Are Mormons and Jehovah's Witnesses Christians? Or Are They Members of a Cult?" You may also want to read: "How to Recognize that You’re Probably in a Cult"
"It is important for us to understand the power of false religion. We’ve all come across it in our endeavor to communicate the gospel to people who are Roman Catholics or Mormons, or Jehovah’s Witnesses or in some other cult, or who are Muslims, or Hindus, or Buddhists, or whatever it is that they might be. We understand the massive captivating power of false religion that captures the mind." - John MacArthur
The Damning Power of False Religion - John MacArthur