Why does it say "Shevor Zero'a Rasha"?
Rashi: Break the arm of Esav (some texts - Amalek) ha'Rasha.
Radak: Break the Rasha's arm and his strength.
Malbim: You Yourself should break his arm [and investigate his evil].
What is the meaning of "va'Ra Tidrosh Rish'o Val Timtza"?
Rashi: When sinners of Yisrael see Resha'im succeeding, their hearts lead them to become evil. When You break the arm of Resha'im, if You come to investigate the evil of sinful Yisraelim, You will not find it.
Radak: The Rasha said "You will not investigate" (verse 13). Show him that You do investigate! If You do so, You will not find a Rasha in the land, for the rest will hear and fear.
Malbim: You Yourself should investigate his evil to punish him, so a Rasha will not be found; sins will cease, and Resha'im will repent.