Will a Muslim who performs religious obligations and good deeds, and is known for the purity of his heart, but mistreats or neglects his parents, be punished?

Insulting parents is considered a sin and an act of disobedience (ʿuquq) for which the perpetrator is punished. Any harm caused to them, whether by word or deed, is an act of disobedience for which the child is sinful, due to violating Allah's command and prohibition regarding honoring them. Allah Almighty says: "And do not say to them [anything that indicates] displeasure, and do not reprimand them but speak to them a noble word. And lower to them the wing of humility out of mercy." The child must repent and seek their forgiveness to escape punishment.