What does John 18:32 mean?

"This," as referenced here, is the fact that Roman soldiers, not Jerusalem's priests, will be the ones to put Jesus to death. This is something Christ has already predicted in detail (Matthew 20:18–19; Luke 18:32). Further, crucifixion would fulfill Jesus' reference to being "lifted up" (John 3:14; 12:32). Later, John will point out by being crucified, not stoned, Christ's death did not involve breaking any of His bones, also fulfilling prophecy (Psalm 34:20) and echoing His role as the Passover Lamb (Exodus 12:46; 1 Corinthians 5:7).

Legal or not, it's all but certain that Jews under Roman rule sometimes executed those deemed guilty of blasphemy (Acts 7:54–60). While technically illegal, it's unlikely Rome cared much about a few Jews killing each other over religion. This is why Jerusalem's leaders attempt to frame Jesus as a revolutionary (John 19:12–15; Luke 23:2). If the Romans kill Jesus, his enemies can direct the people's anger (John 12:17–19) towards their occupiers.

It's the suggestion that Jesus is claiming to be "king," in such a way as to defy Rome, that gives Pilate a reason to try Him in the first place (John 18:33).