How does a Tamim's virtue straighten his path?
R. Yonah: I explained (3) that Tam is perfection of Midos - either via his nature, or he fixed them. This is even if Chachmah did not shine on him to perfect him in recognizing straightness. Here he informs the merit of the Midah of Tamim, and its pleasing intents. It will bring him to straightness of deeds, and remove pitfalls.
Malbim: I explained (3) that Yashar is one whose nature is to go on a straight path. Temimus is standing constantly in one state, without veering from it. If his nature is straight, he will never depart from it. However, if his nature is not straight, he is not assured that he goes on a straight path. Via virtue of a Tamim, he will get used to going in the straight path.
How will a Rasha fall in his evil?
R. Yonah: In the evil of his heart and his bad deeds, he will stumble and fall in the pit of sins. Perforce, Midos will lead to actions - "Shuchah Amukah Pi Zaros] Ze'um Hashem Yipal Sham" (22:14). Rish'aso is the intentional sin of his heart; the same applies to "v'Rish'ah Tesalef Chatas" (13:6).
Malbim: Rasha is the opposite of Tamim. He constantly does evil. Evil will make a Kinyan in his Nefesh, and will fill his way with holes and pits. His Nefesh will fall in them, to the pit of destruction.
