Does a person die a disbeliever if he curses the Lord in a moment of anger, despite his regular prayer and piety, or does Allah forgive him due to the state of anger?

The questioner's statement that a person is diligent in prayer and piety, yet curses the Lord and religion when angry, combines contradictions. Cursing Allah or religion is a major disbelief that expels one from the faith. As for judging this specific person, whether he died as a disbeliever or not, there is no answer, because ruling on a specific individual's disbelief requires the fulfillment of conditions and the absence of impediments, and he might have repented before his death. It is more appropriate to pray for his forgiveness and pardon if he died upon faith.