Why did he say "u'Lecha Hashem Chesed"?
Rashi citing Shocher Tov 4: You pay to a man "k'Ma'asehu" (like, i.e. according to part of his deed), and not the full deed - "Ki Atah Elokeinu Chasachta l'Matah me'Avoneinu" (Ezra 9:13). Radak citing Shocher Tov - an Aveirah is barren, but Tzedakah has Peros - "Imru Tzadik Ki Tov Ki Pri Ma'alelihem Yochelu" (Yeshayah 3:10). An Aveirah is barren - "Ki Gemul Yadav Ye'aseh Lo" (ibid., 11), "Chasachta l'Matah me'Avoneinu" (Ezra 9:13). If a youth sins, he should die. What does Hashem do? He waits until he marries and has children, and He takes one of his children in place of his Aveirah 1 . He collects what is due to Him from His (what He gave). There is no greater Chesed than this!
Rashi: You have the ability to pay to a man according his deed 2 .
Radak #1: It was Chesed when He warned and informed people that He will pay to a man according to his deed.
Radak #2, Ibn Ezra: "Ki" is like 'even though 3 .' Even though You pay to a man according to his deed, at the end, Chesed is to You, for Chesed overpowers - "v'Rav Chesed" (Shemos 34:6).
Radak #3: This refers to Galus. It was Chesed that you made our Galus long, to benefit us in our end. It is Chesed to pay to a man according to his deed in this world, so he will receive his reward in the world to come for the good that he did - "Hen Tzadik ba'Aretz Yeshulam" (Mishlei 11:31).
Radak citing Shocher Tov #2: You do not pay like his intent, rather, k'Ma'asehu. If one planned to sin, but did not do it, Hashem does not write [his sin] until he does it - "Aven Im Ra'isi v'Libi Lo Yishma Hashem" (66:18). If one planned to do a Mitzvah, even though he did not do it, Hashem writes as if he did it. David thought to build the Beis ha'Mikdash. Even though he did not build it, Hashem wrote as if he built it - "Mizmor Shir Chanukas ha'Bayis l'David" (30:1).
Malbim: He saves the oppressed, and supervises the Tzadik 4 [whom the Rasha] oppressed him, for You pay to a man according to his deed - reward to the Tzadik, mercy to the pursued and punishment to the Rasha and the pursuer.
Does Hashem kill the child to do Chesed to his father?! Alshich (Mishlei 31:10, based on R. Shimon ben Yochai): Hashem puts in the sinner's child a Neshamah that needed to return in this world as a Gilgul.
If Rashi means that He could punish a man according his deed, but He punishes less, this is his first Perush! Perhaps he means that Hashem can (and does) reward for Mitzvos. This is called Chesed, for He gave us the means to do the Mitzvos - "Mi Hikdimani va'Ashalem" (Iyov 41:3) - who praised Me before I gave to him a Neshamah? Who built a Ma'akah before I gave to him a house?... (Vayikra Rabah 27:2). (PF)
Radak, Ibn Ezra: E.g. "Ki Am Keshe Oref Hu v'Salachta" (Shemos 34:9).
Seemingly, it should say 'the Rasha who oppressed him', but all texts of Malbim say 'Tzadik'. (PF)
