Crucifixion and the principle of double effect

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.

1. 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!"

2. 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.

3. 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."

4. 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;