Why did he say "if you are small in your eyes"?
Radak: Sha'ul said that the people had compassion, and not himself, but he did not want to stop them.
Toras Moshe (Shemos, Haftoras Parshas Zachor. DH v'Nir'eh): You are small in your eyes, and do not understand why Hashem chose you to be king. If so, you should realize that we cannot know Hashem's reasons. You should have done what His Navi commanded, and not explained it according to your understanding! Refer to 15:15:1:4.
Why did he say "Rosh Shivtei Yisrael Atah"?
Rashi citing Targum Yonasan: The merit of Shevet Binyamin your father caused you [to be king]. They wanted to go first [into Yam Suf] - "Binyamin Tza'ir Rodem" (Tehilim 68:28).
Radak: Even though you are small in your eyes, and you did not want to aggrandize yourself over the nation to stop them, Hashem anointed you to be king over them and guide them in the straight path and stop them from sinning. How did you let them transgress?! They did not do, rather, you - you could have protested, but you did not. This shows that you wanted the matter - you desired spoils and veered to them!
Malbim: Do not say that you could not have opposed the nation - you are king, and the action is attributed to you!
Why did he add "va'Yimshachacha Hashem l'Melech"?
Malbim: You are not like a king that the nation chose; he is drawn after their intent. Hashem anointed you!
