If a person dies without repentance then the matter is up to Allah (the Exalted) whereto they are forgiven or not.
The acceptance of repentance is down to the unbound mercy of Allah (the Exalted) and not up to the things that we think we know about someone. We are not sinless and only the Prophets are perfect and we 'normal' people are prone to trip up sooner or later but when we do we repent and return. We do not become despondent or hopeless we move on and continue.
It does not matter how great your sin the forgiveness of Allah (the Exalted) is greater. Don't ever forget that, ever.
أتبع السيئة الحسنة تمحها
“Following a wrong actions with a good action erases it.” (At-Tirmidhi).
This is also another way out for those in situations where they need to do something to relieve their guilt.
Consider these Quranic passages:
And if you have time then make a supplication for the author who needs to repent more than you do. Be hopeful of forgiveness and never be hopeless. There is always recourse to every problem. Seek repentance from the sins you know, the sins you do not, the open and the secret sins.