Since Vatican 2, they pretty much canonise the deceased and everyone is immediately saying how they're with the Lord and already in Heaven and "went ahead of us" and are waiting for us etc. etc., even in the case of unrepentant public sinners. We all know this.

This is deplored by all traditionalists, and I as well but, what are you supposed to say when someone dies then? If the person in fact lead a bad life and died suddenly, in an instant, with no time to prepare, do you simply say nothing? Do you say a person might be in Heaven only if the person lead a good and virtuous life, or even then, should you still refrain and simply say that you hope God has mercy on his/her soul and never say he went straight to Heaven?

Whether you think the deceased person was a saint or a sinner, just simply say you will pray for the repose of his soul. You can never say "He went straight to Heaven". It is not for us to name his whereabouts. That's up to God.

A great Saint wrote that out of 30,000 souls that past into eternity only five
were saved. Two went straight to Heaven, three went to purgatory. The
rest were damned.
This is what comes into my mind when their is a death. It is a very
dreadful moment for me because I and every living human being will
face their mortality.  

What if you knew the person most likely died in sin? Like, someone who didn't practice the faith at all and lived in sin and died instantly in a head on car crash? Could you still personally pray for the person even though it would technically do no good?

It is one thing for someone dying of an illness or with the last rites and preparing and all that, there you would most certainly pray, but for the sudden death of someone who didn't practice the faith at all?

What if you knew the person most likely died in sin? Like, someone who didn't practice the faith at all and lived in sin and died instantly in a head on car crash? Could you still personally pray for the person even though it would technically do no good?

It is one thing for someone dying of an illness or with the last rites and preparing and all that, there you would most certainly pray, but for the sudden death of someone who didn't practice the faith at all?

From a soul in Purgatory;
You are sad in seeing how God is insulted in Paris, but those poor people do not know what they are doing, in spite of all their blasphemies. Jesus is much more outraged by the sins of those consecrated to Him than by the more violent crimes of those who are not His friends. How many souls whom Jesus has called to perfection remain always worthless because they have not corresponded to Divine Grace. One must put oneself out and constantly check and keep oneself up to a very high standard to be happy in God's service. How very few interior souls there are in the world, and even in religious communities. Each one seeks her own ease and comfort and refuses to be inconvenienced in the smallest thing. And yet God would be so happy (if one may so speak) if they would only love Him without constraint and with all their hearts. If He could only find such joy in this community, what innumerable graces He would shower upon it. For yourself, work your hardest at self-conquest and at loving Jesus, which He has been seeking from you for such a long time. Jesus desires that you should love Him with the love of a child, that is, with the tenderness with which a child seeks to give pleasure to beloved parents. You are still so cold towards Jesus and this is not what He expects of you when He in turn loves you so much.

http://www.purgatory.ca/treasury-manuscript.php

I think we should leave those people we don't know about in the mercy of God and pray for the repose of their souls. Those who apparently don't understand what they are doing are judged differently than those who should know better. I think we should focus on loving God and living in communion with Jesus. One day we will die and I would hope that people will pray for me.

A great Saint wrote that out of 30,000 souls that past into eternity only five
were saved. Two went straight to Heaven, three went to purgatory. The
rest were damned.
This is what comes into my mind when their is a death. It is a very
dreadful moment for me because I and every living human being will
face their mortality.  

This is the season of Advent, when we are obviously reminded of the imminent arrival of Our Lord and Savior, Jesus Christ.

But in reality, that's 2,000 year-old news.

The Catholic Faith teaches us that during Advent we are reminded of the 3 (three) arrivals of Jesus Christ, the first of which is as a babe in Bethlehem (which literally means "House of Bread" - a symbol of the Eucharist). But there are two more arrivals to keep in mind this season.

The third arrival is Our Lord as Judge in the General Judgment when He shall come to separate the sheep from the goats.  But for most of us (hopefully!) there is another arrival that is more imminent.  

The second arrival is when we go to meet our Maker at our particular judgment. And that this is a theme for Advent is clear by reading the Gospel of the First Sunday of this season, which is a parallel Scripture to that of the 24th and Last Sunday After Pentecost, the traditional last Sunday of the year, Matthew 24. Both of these Gospels refer to the end of the world, which for each of us comes at the end of our personal time on earth, our death.

Speaking of death, it is traditional to pray the De Profundis when we hear of someone going to meet their Judge for eternity:

DE PROFUNDIS clamavi ad te, Domine:  
      Domine, exaudi vocem meam:  
Fiant aures tuae intendentes,  
      in vocem deprecationis meae.  
Si iniquitates observaveris, Domine:  
      Domine, quis sustinebit?  
Quia apud te propitiatio est:  
      et propter legem tuam sustinui te, Domine.  
Sustinuit anima mea in verbo eius:  
      speravit anima mea in Domino.  
A custodia matutina usque ad noctem:  
      speret Israel in Domino.  
Quia apud Dominum misericordia:  
      et copiosa apud eum redemptio.  
Et ipse redimet Israel,  
      ex omnibus iniquitatibus eius.  
Gloria Patri, et Filio, et Spiritui Sancto.  
      Sicut erat in principio, et nunc, et semper,  
      et in saecula saeculorum.  Amen.  
 
 
PSALM 129  
(ENGLISH)  
 
(to be said while processing to the refectory)  
 
Out of the depths I have cried to Thee, O Lord: *  
Lord, hear my voice.  
Let Thy ears be attentive *  
to the voice of my supplication.  
If Thou, O Lord, wilt mark iniquities: *  
Lord, who shall stand it?  
For with Thee there is merciful forgiveness: *  
and by reason of thy law, I have waited for Thee, O Lord.  
My soul hath relied on His word, *  
my soul hath hoped in the Lord.  
From the morning watch even until night, *  
let Israel hope in the Lord.  
Because with the Lord there is mercy:  
and with him plentiful redemption.  
And he shall redeem Israel *  
from all his iniquities.  
Glory be to the Father and to the Son, and to the Holy Spirit.  
      As it was in the beginning, is now, and ever shall be,  
      world without end.  Amen.  

I most often simply say: "I'm very sorry". Meaning I am sorry for their loss.

This ^^^.  It's not that difficult.

"I'm very sorry for your loss."