Why will Hashem put a stumbling block in front of him?
Rashi: When he does [evil] covertly, and shows himself to be a Tzadik, I put a known Aveirah in front of him, to reveal his deeds. and when I bring evil (punishment) on him, people will not think that I do improperly. Radak - also Targum Yonasan says so.
Radak: The stumbling block is that he will die in his sin.
Radak citing his father: The stumbling block is that he succeeds in all his deeds in this world. The fruits of his good deeds that he was destined to eat in the world to come, he will eat them in this world. Moshe said "Lo Ye'acher l'Son'o" (Devarim 7:10), like its Targum.
Malbim: This refers to a Tzadik who sinned b'Shogeg. It is "Avel" compared to his virtue. Hashem is meticulous with him like a hair's width, and his Shogeg is considered like Mezid. The Michshol is to consider it a sin for which he will die, even though a Rasha would not be punished for it. Do not think that you need not warn about such a small matter, for he is Shogeg, and his merits will protect him. No! If you will not warn him, he will die for his Shogeg sin, and his merits will not save him - therefore "v'Damo mi'Yadecha Avakesh."
Why will his virtue not be mentioned?
Rashi, from Yerushalmi Pe'ah 1:1: This refers to one who regrets his prior Mitzvos.
Radak: It will not be mentioned in the world to come. It will be mentioned (he will eat the reward) in this world.
Malbim: It will not save him from punishment for his Shogeg sin (refer to 3:20:1:4).
It is written Tzidkaso, and we pronounce it Tzidkosav!
Radak: Tzidkaso is the Klal [of his virtue], and Tzidkosav is the Prat (his virtuous acts).
