Hypothetical: If a Christian commits adultery and dies without ever asking forgiveness of this sin (whether he lives 5 seconds after committing adultery or 5 years), does he go to Hell because he did not repent of this sin? Selected references: Galatians 5:19-21; Ephesians 5:3-6: I Corinthians 6:9-11
Jesus stated, "I tell you, no; but unless you repent you will all likewise perish" (Luke 13:3). Therefore, according to God repentance is not optional.
Whether someone ends up in Hell is not for me to judge. I can only state what the Lord has said. The judgment belongs to God alone. "Therefore judge nothing before the time, until the Lord comes, who will both bring to light the hidden things of darkness and reveal the counsels of the hearts. Then each one's praise will come from God" (I Corinthians 4:5). God's judgment takes into account things neither you nor I have access to. God knows all the facts in a case. He also weighs a person's thoughts and motives. We know that adultery will put a person in hell because God said it would. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). Whether God puts a particular person in Hell because of his unrepented sins is His decision, but He has warned us what that decision will be in advance.