'Mike Norton' (Barry Pepper) Scene Question (SPOILERS)

When Mike Norton drops his wife at the mall he chooses to stay in the truck and have a smoke (in defiance of his wife's command).

As she's sashaying away the scene flashes to the scene of Melquiades' death, and Mike Norton begins to cry.

This was a confusing sequence, for me.

On the surface, it appears that he's rattled because of his accidental killing of the man, but the odd combination of footage

1) his wife walking away, carefree, and
2) Melquiades' blood on his hands

seems to hint at some intertwined emotion connecting the two, for Mike Norton.

Anyone have thoughts on what the filmmaker might have been trying to 'say' in this scene?

Thanks.