How long have you been involved with them?
I was a baptised Jehovah's Witness for almost 15 years before I found out it is all a LIE in late 2004.
Why do you say they are dangerous?
Research the Bible and compare it with what the JW's say about Jesus and you will find out why they are dangerous.
One thing they do is to deny Jesus by twisting the following scriptures all out of proportion and context in both their Bible version and their Watchtower magazines:
Matt 2:11 And when they were come into the house, they saw the young child with Mary his mother, and fell down, and worshipped him: and when they had opened their treasures, they presented unto him gifts; gold, and frankincense, and myrrh.
Luke 24:50-52 And he led them out as far as to Bethany, and he lifted up his hands, and blessed them. And it came to pass, while he blessed them, he was parted from them, and carried up into heaven. And they worshipped him, and returned to Jerusalem with great joy:
John 20:28-29 And Thomas answered and said unto him, My Lord and my God. Jesus saith unto him, Thomas, because thou hast seen me, thou hast believed: blessed are they that have not seen, and yet have believed.
If you become a Jehovah's Witness you are forbidden to have Jesus as your Savior and mediator. The Governing Body of Jehovah's Witnesses becomes your mediator and, in effect they also become your savior instead of Jesus.
They scare the pants off of you by twisting scriptures to say that unless you become a Jehovah's Witness you will be forever annihilated at Armageddon. They predicted Armageddon would occur in 1874, 1915, 1925, 1975.
According to the Bible they are false prophets:
Deut 18:19-22 I will hold accountable whoever does not listen to My words that he speaks in My name. But the prophet who dares to speak in My name a message I have not commanded him to speak, or who speaks in the name of other gods—that prophet must die.’ You may say to yourself, ‘How can we recognize a message the Lord has not spoken?’ When a prophet speaks in the Lord ’s name, and the message does not come true or is not fulfilled, that is a message the Lord has not spoken. The prophet has spoken it presumptuously. Do not be afraid of him.