Why is there great glory in rejoicing of Tzadikim?
Sha'arei Teshuvah (1:18): Tzadikim praise and honor a person for every attribute in them.
Malbim: "Alatz" always refers to Simchah of the Nefesh in spiritual matters. Tzadikim exult in front of Hashem via their virtue. There is great glory in this.
Why does it say "uv'Kum Resha'im Yechupas Adam"?
Rashi: He disguises himself with all kinds of false claims.
Sha'arei Teshuvah (1:18): Resha'im seek people's blemishes and mistakes, to lower them, even if they already abandoned those deeds and repented from them.
Malbim: The intrinsic man is disguised and changes his clothing. [Resha'im's] success is for the animalistic body. They leave the category of man; they are animals in the form of man.
