Why does Iago hate both Otello and Cassio in Verdi’s Otello?

In Giuseppe Verdi’s opera Otello, Iago’s hatred is rooted in professional resentment and a malicious nature.

• Hatred for Cassio: Iago harbors deep animosity toward Cassio because Cassio was promoted to the position of lieutenant, a role Iago believed he deserved himself.1
• Hatred for Otello: Iago’s resentment toward Otello stems from the fact that it was Otello who chose to promote Cassio over him.1

Driven by this bitterness, Iago orchestrates a complex web of deceit to destroy both men. He manipulates Cassio into a drunken brawl to ensure his demotion and subsequently uses Cassio as a pawn to convince Otello that his wife, Desdemona, is unfaithful. Iago’s actions are fueled by a nihilistic worldview, famously expressed in his "Credo," where he declares his belief in a cruel god, reflecting his commitment to malice and destruction.213