The man who eventually became the Mad Violinist (aka the Veiled Violinist) was a Nazi scientist doing sound experiments to try and find a means to destroy buildings with musical vibrations. Using the violin to conduct these tests, he found something far more sinister: a note that could cause the human body to wither and die. Hitting the note and accidentally exposing himself to the resulting sound, has severe effects on him. Although the sound was not powerful enough to kill him, the scientist was disfigured and left with a gaunt appearance. Adolf Hitler himself personally ordered the man to travel to America and use his violin on U.S. military officials.
As the Mad Violinist, he killed a number of prominent military officials across the New York City area, earning the attention of the Patriot, when -- in his civilian identity of Jeff Mace -- the hero witnessed his handiwork. The Violinist was forced to club Mace with his violin, wrecking it.
This proved to be the key for Mace to track down the killer by seeking out recent violin purchases. Confronting the Mad Violinist at his hideout, the Patriot smashed the Violinist's violin and learned of his mission. Not wishing to be taken alive, the Violinist jumped out a window to his death.
Powers and Abilities
The Mad Violinist had the skill to hit the specific key on a violin to generate the so called "death tone" which would kill a grown man or woman, causing them to shrivel up and die. Apparently this tone could not effect children -- which suggests that it has an effect on the biological functions of those who have gone through puberty only.
The Mad Violinist used a standard violin with no visible or identified special adaptation.
A similar violin using Nazi has been seen: The Fiddler.