Whom does this address?
Radak: He speaks to anyone who hearkens to his Musar, like he began the Mizmor "Al Tischar" (verse 1).
Why did he say "Sur me'Ra va'Aseh Tov"?
Radak: Put your hearts and eyes to the way of Tzadikim and Resha'im - Hashem pays them according to their deeds. See - the end of Tzadikim is good, and the end of Resha'im is bad. Therefore, adopt Tzadikim's path - veer from evil and do good. "Sur me'Ra" includes all Mitzvos Lo Sa'aseh, and "va'Aseh Tov" includes all Mitzvos Aseh. Even if one veers from all evil, he is not complete unlike he does good (refer to 1:2:1:2).
Malbim: If you do so, you need not fear evils of the time and its depravity.
Why did he say "u'Shchon l'Olam"?
Radak #1: Via this, you will dwell forever, i.e. serenely. The word 'Betach' is omitted.
Radak #2: This is unlike one who is amidst evil; he has no tranquility or rest. He always moves here and there.
Malbim: You will dwell forever with serenity and trust [in Hashem].
