Here's why:
* He has an affair with Myrtle Wilson: This is a blatant act of infidelity, and he lies to Daisy and everyone else about it.
* He's manipulative and controlling: He attempts to control Daisy and his own narrative, often resorting to lies and intimidation. He belittles Gatsby and tries to convince Daisy to stay with him despite his infidelity.
* He's hypocritical: He criticizes Gatsby for his wealth but is deeply unhappy with his own. He condemns Gatsby for being a "phony" but is himself a major source of deception and dishonesty.
* He's ultimately responsible for Myrtle's death: His reckless driving and refusal to take responsibility for his actions directly lead to Myrtle's tragic death.
While other characters like Gatsby and Daisy also engage in deception, Tom's dishonesty is particularly notable because it's systemic and deeply ingrained in his personality. He seems to have a genuine lack of respect for the truth, making him one of the most morally compromised characters in the novel.