Cycle of bullying

I kept wondering why Shane was so sadistic and especially cruel and personal about how he abused Jack--I even thought there may be a sexual element to it--until I realized that Shane had been bullied in the same way by Tom, bullied to the extent that Tom even faced legal repercussions as a teen. Jack was, of course, the favored son, so it's likely that Tom may have been abused or belittled by their father at one time. And then as I reflected on the scene at Tom's workplace, it occurred to me that Tom was the weakest of the men in the shop. He was clearly afraid of them. In all likeliness, he could face abuse from them as well.

So all of this bullying is a perpetuation of a cycle of abuse. Call it displaced revenge or preemptive attempts at establishing dominance, but I wonder what the filmmaker is saying about this cycle at the end of the film. Much emphasis seems to be placed on Jack looking Shane in the eye, but to what end? In earlier scenes we've already seen Jack's attempts to cobble some sense of retribution--he tags Shane's garage and throws a rock at him. But in the last scene, in a most brutal attack, there's much emphasis on Jack standing up, looking Shane in the end, and not running away. It's as if he's saying, Kill me if you have to, but I'm not running away today.