Why did he say "Ach ba'Chalakos Tashis Lamo"?
Rashi: All the good that comes to them (is flattery) - in the end, "he made them fall disastrously."
Radak: What You gave to them in this world, was to make them fall in the world to come. This is like one who goes on a smooth path. It seems to him that the path is good, because it is smooth. Soon his feet slip and he falls disastrously. So in this world, their good is not for a long time, and also in their death their remembrance is lost, and none of their good remains for their children.
Malbim: This is like one who dug a pit to trap a Chayah, and he smoothed the path that leads to the pit. Even though the Chayah going on the smooth path thinks that it is to make its going easier, in the end it is revealed that via the smoothness, it fell to calamity. So via the serenity and success [of the Rasha] in this world, he will die an eternal death and fall to Gehinom. Divine straightness obligates that no creation lose its reward. Also one who was evil all his days, and did one Mitzvah, he must receive his reward for that Mitzvah. Similarly, a Tzadik who did one Aveirah must receive its punishment 1 .
Malbim: Hashem saw that the eternal spiritual world in which a person lives after his death, it is too honorable [and therefore improper] for a Rasha to be rewarded there or a Tzadik punished there. The reward and punishment there will be eternal and true, and not temporary and imagined. Therefore, a Rasha's reward and a Tzadik's punishment must be in this physical world. The Rasha's success and the Tzadik's afflictions have no existence in intellect, only in semblance. The physical world is a world of sensation dependent on time and place. The good and evil that people sense in it are only imagined, and temporary. They apply to the lowest part of man (animalistic), but not to his primary essence (his Neshamah).