First it is important to point out that Jehovah's Witnesses do not believe that Jesus is *just* an angel. Scripture suggests that he is *the* ARCHangel.
The Bible refers to Michael as “the ARCHangel.” (Jude 9) This term means “chief angel.” Notice that Michael is called THE archangel. Not only does this suggest that there is only *one* such angel, but also the term “archangel” occurs in the Bible only in the singular, never in the plural. So when 1 Thessalonians 4:16 states: “For the Lord himself shall descend from heaven, with a shout, with the voice of the archangel”, (ASV) the voice of Jesus is described as being that of the archangel. This Scripture therefore suggests that the name 'Michael' applied to God’s Son before he left heaven to become Jesus Christ and also after his return.
For more, see: Who Is Michael the Archangel?
http://www.jw.org/en/publications/books/bible-teach/who-is-michael-the-archangel/
and
Michael (Insight on the Scriptures)
http://wol.jw.org/en/wol/d/r1/lp-e/1200003035
Jehovah's Witnesses today are not (and have not been) the only ones to understand Michael to be the archangel who came to be known as Jesus on earth. From early Christian writings until now there have been many who have believed this:
Quotes From Scholars Concerning Jesus as Michael the Archangel
http://examiningthetrinity.blogspot.com/2011/06/scholars-quotes-concerning-jesus-as.html
Trinitarians obviously have problems with this because an "angel" to them is a subordinate term thereby reducing Jesus to a status lower than God Himself.
A large problem is that many do not have the proper understanding of what the literal definition of "angel" in the Bible is. Both the Hebrew mal·´akh´ and the Greek ag´ge·los literally mean “messenger.” When spirit messengers are indicated, the words are translated “angels,” but if the reference definitely is to humans, the rendering is “messengers.” (Gen. 16:7; 32:3; Jas. 2:25; Rev. 22:8) Whether human or angelic messengers are meant can be determined by the context.
When Jesus was on earth, he was God's foremost messenger. He was called the "Word" because he was God's Spokesman:
“I have not spoken out of my own impulse, but the Father himself who sent me has given me a commandment as to what to tell and what to speak. . . . Therefore the things I speak, just as the Father has told me them, so I speak them.” (John 12:49, 50; 14:10; 7:16, 17)
The Bible clearly shows that Jesus is not God in hundreds of different ways. So the over-the-top persecution of Jehovah's Witnesses by trinitarians over whether or not Jesus is Michael the Archangel is really an overreaction and demonstrates a clear ignorance of Scripture and History.