Lighter, you seem to be making the assumption that the Union soldiers were firing upon the Confederates as they advanced. But it doesn't seem so, from the tone of Pickett's narrative. It sounds as though the Unionists had to get within range before they could fight, and that they were marching to what everyone – on both sides – knew was their deaths. Pickett calls it "suicide" which suggests a fatal inequality of power, giving more an image of doomed determination than of aggression. Your own well-worded description struck me the same way: "Ranks crossing a broad field had to advance slowly until nearly to the objective, and once enemy fire began to take its toll, the ranks would wobble and mix together in clotted masses as the survivors pushed on unevenly through clouds of smoke… There is just no time for charity toward the enemy when he's trying to kill you." But it looks like the ranks crossing the field were getting killed rather than doing the killing. If you're the cat and the other guy is the mouse, you can sometimes afford to be magnanimous.   

Also, how practicable was it in the 1860s to "rain cannon fire" while in motion? (That's not a rhetorical question – I really don't know; but the little I remember about large 19th-century weapons makes me think they were treacherously awkward to manoeuvre, even if the troops had the luxury of smooth ground.) The Unionists had to keep moving. All the Rebels had to do was wait for them and pick them off.

I used to work in the rare documents and letters department of an antiquarian book shop (Goodspeed's, Beacon Street, Boston; back in the early 70s) and I once had a packet of letters from a Civil War soldier land on my desk. I can't quote you chapter & verse, but there was a spirit of weary compassion about them - no gloating or jeering at the enemy, whose sufferings he obviously understood only too well.

As has already been pointed out, extraordinary times can call forth extraordinary responses.