How does Nick's statement that "I couldn't forgive him or like him but I saw that what he had done was, to him, entirely justified" relate to the...

Here, when Nick bumps into Tom after Gatsby's funeral, he reflects on Tom and Daisy and how "careless" they are: how they ruin people, break things, and basically do whatever they want, and then they simply retreat back to their giant home and piles of money without being held responsible or accountable for the damage they've done. Nick realizes Tom believes allowing Gatsby to be held accountable for Myrtle's death (despite the fact that Myrtle was rushing out to speak to Tom and that Daisy was driving the car that hit her), is completely justified because this is the way life has always been for Tom and Daisy. They think they can do whatever they want because they've never been held responsible for the repercussions of their actions, and Nick feels as though he is "talking to a child" because it seems like there is no chance of making Tom understand what he's done. 

Nick then describes Tom walking into a jewelry store to purchase, perhaps, a pearl necklace or a pair of cuff links.  Tom's actions show just how completely unfazed and unaffected he is by what has happened. His life just goes on as usual. It is such a commonplace thing for him to purchase luxury items — that is the kind of action that defines him.  People like Tom and Daisy seem beautiful, but they really have a corrupted, false kind of beauty: their facades are beautiful, but so ugly underneath. This is what makes it meretricious and vulgar.