Greed.
McTeague, the titular character, is characterized by his greed. His best friend, who seems less selfish than Mac, proves to have his own greedy side when Trina discovers her winning lottery ticket. The money could have been Marcus's if he had followed through with his intentions for Trina. Trina proves to be a greedy person in her own way, becoming obsessed with her money and keeping it all from McTeague.
Jealousy.
Another major theme is jealousy. Trina has the money she has always wanted, and Mac comes to her like the Prodigal Son, asking for money. Marcus constantly harasses the couple because of his own jealousy, and eventually, their constant jealousy leaves everyone dead.
The corruption of character.
The main theme of the novel seems to be the decay of moral character, which is exacerbated by San Francisco's dog-eat-dog economy. When the reader meets the characters, they seem pleasant. A best friend deigns to let his buddy have the girl they both want. Trina and Mac have a pleasant marriage.
But Trina's obsession with money (keeping and saving it) corrupts her to the point of delirium (she literally rolls around in her money like a cartoon character). Marcus's unbearable jealousy leaves him constantly picking fights he can't win, and Mac's newfound poverty brings out his abuse, violent side. Eventually, he murders Trina and Marcus. The effect is that the characters are progressively spiraling into darker and darker mindsets until their selfish, sociopathic desires are met.
