Simply, the Torah is telling us that Hashem will be angry as a result of our iniquity. Horav Nissan Alpert, zl, makes a novel homiletic exposition to the pasuk, implying a valuable lesson. Hashem sees the iniquity, but that alone does not cause Him to become angry. After He sees how His children, Klal Yisrael, act when they do not get their own way, when they do not get their hearts desires, He is provoked to anger. It is one thing to make demands, to entreat Hashem for favors and hope that they are fulfilled. How do we have the audacity to sin, to perpetrate our evil actions, and still make demands on the Almighty? Furthermore, when we do not receive what we want we complain bitterly, impugning Hashem. Does this attitude represent gross chutzpah or is it merely human nature?
How often do we find people bemoaning their fate, expressing their complaints to the Ribono Shel Olam, while they continue to flaunt their selective form of religious observance? Perhaps if we were to open our eyes and take stock of our hypocritical behavior, we might see what Hashem perceives all of the time.