The difference between someone dying attached to a mortal sin and someone that has repented but still has the guilt of that sin is huge, one to Hell and the other to Purgatory followed by Heaven.
But the difference between someone dying attached to venial sins which they haven’t repented (they might not know what they are) and a man that has repented of his venial sins but is still attached to the guilt of them both go to Purgatory.
So what is the difference between attached to venial sin and attached to the guilt of venial sin when you die?
A person cannot be “attached” to the guilt of venial sin - only to committing it again in the future, which means no real repentance in his heart. The guilt of venial sins committed in the past is taken care of by indulgences. and/or voluntary penances.
As for not knowing what his venial sins are, how can he repent of them? But God is just, and if there is any penalty due for these unknown sins, it will be His to judge.
You must first recognize the difference between venial sin and mortal sin. It seems to me like you know it, but…
“‘Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.’” (Catholic Church 1857)
With that in mind, I’m pretty sure you’re going to have to deal with major guilt after repenting.
“One commits venial sin when, in a less serious matter, he does not observe the standard prescribed by the moral law, or when he disobeys the moral law in a grave matter, but without full knowledge or without complete consent.” (Catholic Church 1862)
If you didn’t realize what you did was a sin, wouldn’t you feel a lot less guilty than a person who committed a mortal sin?
A mortal sin is a much more worse sin than a venial one. Therefore, once you see how bad a mortal sin really is, you’ll realize why it might seem “unfair” (and know it’s really not) for the people to go where they end up like you said in your example.