How is this connected to the previous verse?
Rashi: Those who guard his Edus are happy if they have all these attributes.
Why does it say "Af"?
Radak #1: This means all the more so, like "Af Shochnei Vatei Chomer" (Iyov 4:19). If they guard His Edus, i.e. Mitzvos Aseh, all the more so they will not do Avlah (transgress Lavim).
Radak #2: This means also; also not doing injustice is considered like "bi'Drachav Halachu" (doing an Aseh) 1 .
Radak #3: Since they seek Hashem with all their heart, they surely did not do evil. An Aveirah will not come upon them - one who seeks to purify himself, [Shamayim] helps him (Shabbos 104a).
Malbim: The Edus help not only for investigation, rather, also for action. They teach Hashem's conduct, to punish all evildoers, e.g. Dor ha'Mabul, Sedom and Egypt. Via this they guard and do not do injustice.
Why does it say "Lo Fa'alu Avlah"?
Malbim: Pa'al is engagement in a matter. They do not even engage in a matter with Avlah.
What is "Derech"?
Radak (1): Derech is fixing Midos. Just like Hashem is gracious, you should be gracious. Just like Hashem is merciful, you should be merciful. The same applies to other Midos.
Why does it say add "bi'Drachav Halachu"?
Rashi (from Shocher Tov 1:7): Even if they did not do injustice, their reward is complete only if they went in His ways. Also above (34:15), it says "Sur me'Ra va'Aseh Tov." Even though he veers from evil, he is complete only if he does good.
Radak: Refer to 119:3:2:2 and the note there.
