1)
When will the Tzadik rejoice?
1.
Radak: When Hashem's anger comes against [the Resha'im].
2)
How does he see vengeance?
1.
Malbim: The verse depicts vengeance as if it is a living entity that kills the Rasha, and afterwards, it washes its feet in his blood. It says "Chazah", for it is a concept 'seen' in the heart, and not with the eyes.
