1)
What is "Samim"?
1.
Radak: I did not doubt [His conduct] when Sha'ul pursued me. I did not refute His prophecy, and say 'how could Kel command that I be king, and Sha'ul pursues me every day to kill me?!' Also "Tamim Tihyeh Im Hashem Elokecha" (Devarim 18:13) forbids questioning His Midos.
2.
Malbim: His body and intellect lack the power to do something against Mitzvos Hashem.
2)
Why is he guarded from sin?
1.
Radak: He was guarded from striking Sha'ul.
2.
Malbim: He reached Tamim, i.e. the nature of his body and intellect converted to Kedushah and Taharah, and he has no investigation, leaning or other intent in Avodas Hashem. Automatically, he is guarded from sin, for he does not desire it at all.
