In the moment we realize that M. Lantin has been duped, so to speak, by the wife he so loved, we have sympathy for him.  He's in an embarrassing situation--having others know he's been cuckolded by his wife not once but presumably many times, even if it's the same man--and yet he must persevere.  In other words, as a practical matter he really can't just walk away from his shame.  He needs the money, but he also needs the purging which getting rid of the jewelry will provide.  We feel this way about him until he faints on the sidewalk.  Now, once he turns angry and bitter, our feelings toward him change.  While we may understand, we can't and don't condone such ugliness.  It's bad enough that, in the ironic reversal at the end when he gets an honest wife and is miserable, we're kind of glad.