What is "Tamim"?
Radak (Tehilim 18:24): 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.
Malbim (Tehilim 18:24): His body and intellect lack the power to do something against Mitzvos Hashem.
In what way is he guarded from sin?
Radak (Tehilim 18:24): He was guarded from striking Sha'ul.
Malbim (Tehilim 18:24): 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.
