Crucifixion and the principle of double effect Crucifying Jesus is obviously morally wrong. Being silent on another person's sin is also a sin. God is sinless.

How does, from Roman Catholic perspective, one explain the fact that Jesus didn't stop the Romans from crucifying him, therefore allowing for their sin?

I suppose it has something to do with the principle of double effect, but I don't understand how it works with the "morally evil act can't be used as means" thing.
Jesus' obedience to his Father's will was more important than one more example  of people sinning

His sacrifice paid for their sins, because it pays for all of mankind's sins.  Looked at through the lens of the Satisfaction Theory of Atonement Jesus was about to pay for their sins himself.  The sins of the Roman were but a few of the sins of mankind that called for satisfaction.  


  The satisfaction due to God was greater than what all created beings
  are capable of doing, since they can only do what is already required
  of them. Therefore, God had to make satisfaction for himself. Yet if
  this satisfaction was going to avail for humans, it had to be made by
  a human. Therefore only a being that was both God and man could
  satisfy God and give him the honor that is due him.  


Jesus' obedience is the opposite of sin.   


  CCC 1850 Sin is an offense against God ... Sin is thus "love of
  oneself even to contempt of God." 1(~St. Augustine). In this proud
  self-exaltation, sin is diametrically opposed to the obedience of
  Jesus, which achieves our salvation.(Philippians 2: 6-9)


Note that this answer is a frame challenge, since from the Catholic perspective none of this alleged "double principle" in play.      


The Agony in the Garden of Gethsemane

As recorded in all four Gospels, after the Last Supper, Jesus went into the Garden of Gethsemane to pray before he was arrested.  The Gospels present him as confronting his fate, during which prayerful time he asks his Father to take this cup (his looming passion / crucifixion) away.  In the end, he submits to his Father's will.  "Not my will, but yours."  Being obedient to God is not a sin, it is a virtue.  


  (Matthew 26:42). "My Father, if it is possible, let this cup pass me by. Nevertheless, let it be as you, not I, would have it." Then, a little while later, He said, "If this cup cannot pass by, but I must drink it, your will
  be done!"   

Agnus Dei  

"Agnus Dei, qui tollis peccata mundi, miserere nobis*
(Lamb of God, Who takes away the sins of the world, have mercy on us)    

In the Roman Catholic liturgy, the Agnus Dei is a reaffirmation that Jesus, the Lamb of God, takes away the sins of the world.  This is drawn from Scripture:  John 1:29    


  The next day he saw Jesus coming toward him and said, “Behold, the
  Lamb of God, who takes away the sin of the world."    


That's what Jesus did on the cross: since the Romans were of the world, their sins were an included case.  That's Jesus' role, which he accepts during the Agony in the Garden.  From his arrest to his crucifixion (the passion) Jesus was not able to stop the railroad job that was coming his way.  He knew that ahead of time, based on his prayer in the Garden, and remained obedient to his Father's will.  Trying to stop it would have been in direct opposition to God's will, and thus a sin.   
Their sins (the Romans') were paid for by Jesus' sacrifice, as was everyone else's.  From the Catechism. 


  CCC 457 The Word became flesh for us in order to save us by
  reconciling us with God, who "loved us and sent his Son to be the
  expiation for our sins": "the Father has sent his Son as the Saviour
  of the world", and "he was revealed to take away sins"  
  
  God takes the initiative of universal redeeming love
  CCC 604 By giving up his own Son for our sins, God manifests that
  his plan for us is one of benevolent love, prior to any merit on our
  part: "In this is love, not that we loved God but that he loved us and
  sent his Son to be the expiation for our sins." God "shows his love
  for us in that while we were yet sinners Christ died for us."

Forgive them Father, for they know not what they do (Luke 23:24)

Even before he was nailed to the Cross, Jesus recognized their sin in putting to death an innocent (per Luke 23:4, even Pilate recognizes Jesus' innocence).  

So does he whine about it?  No.  He not only remains obedient to God, He makes an appeal to his Father, asking for them to be forgiven because they did not know what they were doing.  That's (1) loving your enemy, and (2) loving your neighbor as yourself --  at the same time.  If there's any principle of double effect going on, it is one of love, not sin.     




The question you ask has some odd assumptions.  It is hardly a sin to be unable to stop someone from sinning when you have not the ability to do so.  (I can't stop someone in Pakistan from committing murder: I haven't the means to do so).  What he did is what he could do, based on his obedience to his Father: take their sins to himself, as was his role, as he takes the sins of all of mankind.  



Expiation, featuring Jesus as victim of expiation for human sins, is covered in CCC articles 457, 604, 615-16, 1476, 1992;