Is it the case that whoever dies while committing a sin will be tormented in Hellfire, even if that was the first time he committed that sin?

If a Muslim dies persisting in a major sin—whether they died during the act or after it without repenting—their fate is subject to Allah's will. If He wills, He will punish them, and if He wills, He will forgive them. No specific Muslim can be definitively condemned to punishment, no matter how great their sins. This is the belief of Ahl al-Sunnah wal-Jama'ah (the People of the Sunnah and the Community). Repeatedly committing a sin and persisting in it magnifies the sin, making it different from a one-time act without recurrence.