Do you know any songs that have a cool, dark violin part?

Are you looking for a violin solo? Try the Bruch Concerto in g minor, first movement. There was a documentary on Hitler a few years ago and they used this piece as his theme. IT is not to sad, not to angry, but it's dark. And very hopeless.

Try some Bach :
http://www.youtube.com/watch?v=ET100brwy...

http://www.youtube.com/watch?v=q8E4z4VQV...

http://www.youtube.com/watch?v=q8E4z4VQV...

or how about Vitali :

http://www.youtube.com/watch?v=AloBa9SPM...

There is an extraordinary use of the violin(s) in the final scene of R. Strauss opera "Salome".

After Herod has ordered that John the Baptist's head should be brought to her on a platter, she hovers over the edge of the cistern, wherein the prophet is incarcerated, waiting for the executioner to deliver it up to her.

During this time, the strings are "pinched", not plucked; and it makes for a most "hair raising" experience for the listener.

Wotan

Also, the music by Secret Garden do have a lot of violin parts, many of them sad and slow...

I'm not sure if it's what you're looking for, but I thought of "Fratres" by Arvo Part.

Also try Ravel's Violin Sonata.

well, i really like carmen fantasy by sarasate
it sounds like this
http://www.youtube.com/watch?v=aXqrl2gXG...
and this is the second part (cuz its long)
http://www.youtube.com/watch?v=zJXVPE2a1...
its a really good song with an excelent violin solo
i really want to play it, but im not advanced enough. iv been playing since iv been 4, but in order to be able to play this song, it looks like i have to work harder (o>.<o)