The best friends at odds
When Mac and Marcus meet (notice their names' similarity) they are best friends. Marcus cares enough for Mac to let him pursue Trina, even though Marcus originally intended to pursue her himself. But eventually, the niceties die off, and the real relationship is exposed. Marcus is greatly jealous that Mac got the life Marcus wanted. Therefore, the best friends become mortal enemies.
The lottery jackpot
It probably would have been easier to let Trina go if she hadn't immediately won the lottery. Marcus didn't know that when he gave up the girl, he gave up a life of luxury. Except he's wrong. Mac doesn't enjoy a life of luxury. Instead Trina nickels and dimes their finances and leaves Mac with just as much financial debt and struggle as he originally had. The lottery represents something, but it's not good fortune. In fact, Trina loses her mind about it, and ends up losing her life when her jealous husband decides to rob her.
Trina's family moving away
When Trina gets married, her mother is part of the gang. Because Trina's family was also in San Francisco, she felt a sense of normality and safety in her life. But this is all removed from her when Trina's entire family leaves. Trina is forced to do marriage alone, without the support she depended on.
Mac's unlicensed, illegal dentistry
When Marcus gets his revenge, the reader learns that McTeague's dentistry practice is a sham. He has no educational background in dentistry, and his practice isn't licensed. It seems Mac built his entire life as a house of cards. This is ironic on multiple levels, especially since the revelation shows Mac's hidden willingness to extort innocent people—which is to say he is a psychopath.
The murders
Mac isn't the sharpest tool in the shed, in terms of his intelligence. This leaves him in a world of physical violence. He fights Marcus a few times throughout the novel, even breaking his arm, and eventually, his abusiveness comes out in the marriage as well. His character development is complete when he kills his wife to steal their money, and then he murders Marcus in Death Valley.
