When someone nearby on the train who has the volume on their portable music device cranked up so high that I can hear it bleeding out from their earbuds, even above the sound coming through my headphones, I appreciate it when he bobs his head in rhythm with the screeching beats. It allows me to identify the target for my silent disdain, rather than merely wishing ill upon everyone in the car (that's not right).
The way I see it: If you're going to be an oblivious asshole, at least have the decency to be open about it.
That way I know for whom to buy a pair of headphones that don't suck so bad and actually keeps the sound focused on just him. Then with the full volume blasting into only his ears, it will expedite the inevitable deafness that will make pointless him wearing any headphones at all. However, after I've done that, if he could hurry up and lose his hearing quickly enough that I can return the headphones, it would be uncharacteristically considerate of him, and I'll no longer consider him worthy of hate.
I'll even teach him the sign language I know. (Let's hope he wants to learn how to order beer, wine, whiskey, or vodka.)
There's always a way to resolve these situations without resorting to violence.