1)
What is the meaning of "Nichamti"?
1.
Rashi: A thought entered My heart, regarding the kingship that I put upon Sha'ul.
2.
Malbim: When Hashem 'regrets' an act, it totally nullifies it. The existence of everything depends on Hashem's desire - if He does not want it, it is as if it does not exist, like "Nichamti Ki Asisim" (Bereishis 6:7). This comes with a decree, that Teshuvah cannot overturn.
2)
Why did Hashem regret making Sha'ul king?
1.
Malbim: He turned away from Me. He sinned in thought, amidst lack of Emunah in Hashem's word and His Navi. Such denial is worse than sin amidst lust or one's nature. Also, he did not fulfill My word.
