Why did he say "Ki Sihyu Li ha'Yom l'Satan"?
Malbim: If I kill him, people will say that I bear anger for my enemies, and I will punish all who rebelled, and they will not return to me. 1
Could he not publicly pardon all who rebelled, and announce that Shim'i alone will be punished for cursing? (PF)
Why did he say "ha'Yom Yumas Ish b'Yisrael"?
Malbim: It is as if David is a new king. The custom is that when a king rises to power, he pardons from death all prisoners. Surely one should not decree death on this day!
Why did he say "Ki ha'Yom Ani Melech"?
Rashi: Before, I thought that a great person like him disgraced me because my kingship ceased, and Hashem told him to curse me. Now that I see that he regrets, I know that I am king.