וְהִנֵּ֣ה עִ֠מְּךָ שִֽׁמְעִ֨י בֶן־גֵּרָ֥א בֶן־הַיְמִינִי֮ מִבַּחֻרִים֒ וְה֤וּא קִֽלְלַ֙נִי֙ קְלָלָ֣ה נִמְרֶ֔צֶת בְּי֖וֹם לֶכְתִּ֣י מַחֲנָ֑יִם וְהֽוּא־יָרַ֤ד לִקְרָאתִי֙ הַיַּרְדֵּ֔ן וָאֶשָּׁ֨בַֽע ל֤וֹ בַֽיהוָה֙ לֵאמֹ֔ר אִם־אֲמִֽיתְךָ֖ בֶּחָֽרֶב׃
“You must also deal with Shimei son of Gera, the Benjaminite from Bahurim. He insulted me outrageously when I was on my way to Mahanaim; but he came down to meet me at the Jordan, and I swore to him by the LORD: ‘I will not put you to the sword.’
9
וְעַתָּה֙ אַל־תְּנַקֵּ֔הוּ כִּ֛י אִ֥ישׁ חָכָ֖ם אָ֑תָּה וְיָֽדַעְתָּ֙ אֵ֣ת אֲשֶׁ֣ר תַּֽעֲשֶׂה־לּ֔וֹ וְהוֹרַדְתָּ֧ אֶת־שֵׂיבָת֛וֹ בְּדָ֖ם שְׁאֽוֹל׃
So do not let him go unpunished for you are a wise man and you will know how to deal with him and send his gray hair down to Sheol in blood.
עתה רצה לומר אני נשבעתי ומחלתי לו ,ועתה אם יחטא לך
And now: it means, "I swore, and I forgave him," but, "if he sins against you..."
(Malbim)
Dovid HaMelech knew that Shimi ben Gera needed to be punished, since he technically was mored b'malchut, so a) why does he forgive him, and b) then tell Shlomo not to let him go unpunished?
This is where the Malbim comes in. One beautiful thing we can take away from this Malbim is the idea of second chances. Dovid Hamelech forgave Shimi ben Gera, something which most people would've agreed that he didn't deserve. Now, he's telling Shlomo only to punish him if he really has to.
Dovid gave someone who cursed him soundly in public a second chance and forgave him- and we can't give our friend one? We can't forgive our sister for wearing the shirt we wanted to wear that day? Whenever someone does something to us, we need to remember what Dovid said- "If HaShem didn't want him to curse me, he wouldn't have." If HaShem didn't want us to be upset, he wouldn't have made our friend say that. It's all part of a plan.
On another level, this can give us chizuk in terms of our relationship with HaShem. So many times, we mess up and we think, "That's it. I messed up so badly." We may even think that we can't do teshuva. But if Dovid could forgive someone who openly cursed him and embarrassed him publicly, if he could give someone he barely knew a second chance, קל וחומר HaShem who loves us and has infinite patience. Not only will he give us a second chance, he'll give us a third and a fourth, even a twentieth chance.