Why does it say "Shemar Tam"?
Rashi: Look at the ways of the pure people, to learn from their deeds.
Radak: A Tam [and Yashar] are different than the Rasha mentioned in verses 35-36.
Malbim: This commands the listener to be pure, and not heed the success of Resha'im in this world, for there is another world in which he will be judged for everything hidden.
Why does it say "u'Re'eh Yashar"?
Malbim: There you will see Hashem's straightness - payment of the Tzadik and Rasha.
Why does it say "Ki Acharis l'Ish Shalom"?
Rashi: If he does not have a beginning, he has an end. This is not so for sinners (verse 38). Radak - "l'Ish" teaches that for a man like this, You see that his end is Shalom.
Malbim: A Tzadik has an end in the world of Neshamos, which is the end of man.
