Albert Barnes Commentary John 18:32

"that the word of Jesus might be fulfilled, which he spake, signifying by what manner of death he should die." — John 18:32 (ASV)

That the saying of Jesus, and so forth. Namely, that he would be delivered into the hands of the Gentiles and be crucified (Matthew 20:19). Neither of these things would have happened if he had been put to death in the way that the Jews first contemplated (Matthew 26:4).

Even if it is granted that they had the power, in religious cases, to do this, it still would not have been done, as Jesus predicted, by the Gentiles. And even if it is granted that they had the right to take life, they still did not have the right to do it by crucifixion.

This was particularly a Roman punishment. And so it was ordered, in the providence of God, that the prediction of Jesus in both these respects was fulfilled.