Bach: 'Erbarme dich' St Matthew Passion - Andreas Scholl (and 'Ruht Wohl' St John Passion)

The subtly rhythmic “Erbarme Dich” is as meditative as “Ruht Wohl” but far more melancholy. It is no lullaby. And yet how could anything be sadder than mourning the dead? 

(...) The secret of the aria lies not in its melody but its rhythm. The time signature is 12/8, which is that of most slow blues ... Bach has 12 beats to play with per measure, each one worth an 8th note (yep, that’s why it’s called 12/8). Like any good bluesman, he arranges them in runs of triplets. His runs go down the natural minor scale of B (recall that natural minor scale = scale of relative major, which here would be D). Hamari - the singer - sustains that B over 9 beats: GGG-F#F#F#-EEE. (By all means, go ahead and count the 3 triplets by tapping gently on your keyboard 9 times.) This is just like a jazz walking bass line. But instead of Charles Mingus, we’ve got cellists plucking the strings of their instruments to evoke the tears flowing down Peter’s cheeks. (We know that from the prior recitative and the fact that Bach was always big on sound imagery ....) The descending line is relentless. The walking bass goes down and down and down, then comes up for air only to resume its plunge.

Yehudi Menuhin was crazy about the violin obbligatos. He called the “Erbarme Dich” solo the most beautiful piece of music ever written for the violin. To me, the genius of the music’s pathos is that it isn’t the slightest bit manipulative (that minor-mode affliction so common in popular music.) Here, it’s about the sting of remorse. I find the humility and intimacy of the music almost overwhelming. Like a blues tune, it is a deeply personal statement, not a collective one. 
— From 'Erbarme dich' Tiny Revolution website by Bernard Chazelle, http://www.tinyrevolution.com/mt/archives/002903.html