What criteria would a religious cult have to meet to be classed as evil?

All cults are evil not just the JW as you point out. They are generally run by false prophets who deceive people with supposedly miracles (Mat. 24:24; Rev. 13:14), good words and fair speeches (Rom. 16:17-18), and of course, just down right lying (Jer. 14:14). False prophets generally have a very positive outlook toward man, unity, love, science, education, integration, and world peace, but they have a very negative view toward the infallibility of scripture, salvation by faith in Christ alone, the Blood Atonement, the Deity of Christ, and the Pre-millennial Second Coming of Christ. Isaiah 8:20 and Deuteronomy 18:20-22 are great verses to keep in mind when trying to discern between true and false prophets. Also remember Matthew 7:20: "Wherefore by their fruits ye shall know them."