Bava Basra 14b: David and ten other Chachamim composed it - Adam, Malki Tzedek (Shem), Avraham, Moshe, Heiman, Yedusun, Asaf, and Korach's three sons.

Rashi: Ten men - Adam, Malki Tzedek, Avraham, Moshe, David Shlomo, Asaf, and Korach's three sons. Some say that Yedusun is Adam, and some say that he is called so due to the Datin (laws) and judgments that passed over him ¹ and Yisrael (Shir ha'Shirim Rabah 4:3).

Rashi: This is one of 10 expressions of song in this Sefer. The others are Nitzu'ach, Nun, Mizmor, Shir, Hallel, Tefilah, Brachah, Hoda'ah and Hallelukah. They correspond to the 10 authors (refer to 1:1:1:2). A man's Ashurav (perfection) and praise are that he did not go [in the counsel of Resha'im], for since he did not go, he did not stand, and since he did not stand, he did not sit (refer to 1:1:6:1)...

Radak: Ashrei is always plural, for a person is not happy with one good or success found in him, only if many good things are in him. Then he is called happy. David put in this Mizmor what our enemies should do in this world, and the good reward for Tzadikim and the punishment of Resha'im.

Radak (2, citing Brachos 9b, 10a): Every Mizmor that was dear to David, he began and ended it with Ashrei. One opinion holds that "Lamah Ragshu

Radak: This is like "Sur me'Ra va'Aseh Tov" (34:15). From youth, one grows the way of the lust of the world, to eat, drink and be merry - "Ki Yetzer Lev ha'Adam Ra mi'Ne'urav" (Bereishis 8:21). When he matures and begins to recognize good and evil, our verse commands him to veer from the way and counsel of Resha'im. They are zealous to acquire money and their hearts' desires in this world, whether good or evil. They steal, rob and kill for money. The essence of a Rasha is anxiety of heart - "v'Hu Yashkit u'Mi Yarshi'a" (Iyov 34:29). "Derech" is not the road, rather, one's custom, engagement and deeds, like "v'Hodata Lahem Es ha'Derech Yelchu Vah" and similar verses.

Radak: It is good to gather money and be happy. People err greatly in this. A person is pleased with what looks good; he does not look at its end.

Malbim: Resha'im sin b'Mezid, both Bein Adam l'Makom and Bein Adam l'Chavero. The verse mentions their counsel; their sins are planned. Chot'im do not have counsel to sin. They sin due to desire.

Radak citing Avodah Zarah 18b: If he went, in the end he would stand; if he would stand, in the end he would sit; if he would sit, in the end he would scoff.

Radak: He did not delay, strive [to be close] with them or stand in their company, lest he learn from their deeds. There are three situations of man - going, standing, and sitting, which includes lying. Also, lying is usually for sleeping, and when one is sleeping, he does not do good or evil.

Malbim: He does not sin due to desire. Not only he does not go in their way, to sin constantly - he does not even stand in their way temporarily (sin occasionally).

Radak: They are crafty to do evil. They are haughty and speak bad about people, and find faults and blemishes in them, and reveal secrets to each other. Here it discusses idle people who sit on the corners.

Malbim: They sit on the corners and do not do evil actions, but they do not do good. They pursue only folly and scoffing and do not engage in Torah.

Lev Eliyahu (Shemos p.146): Even if he fulfills Mitzvos and carefully avoids Aveiros, if his primary thoughts are about bodily desires and follies of his time, this is a Letz!