What does John 19:9 mean?

Pilate had ample reasons to avoid putting Jesus to death. The conflict with religious leaders is obviously personal (Matthew 27:18). Jesus is clearly not a political threat (John 18:36–37). Pilate's own wife has expressed concerns (Matthew 27:19). To make matters worse, Jewish leaders have now told Pilate that Jesus claims to be the "Son of God" (John 19:7). This is true (Matthew 26:63–66), but instead of considering it blasphemy, Pilate is afraid it might be true (John 19:8).

In Roman religion, it was possible for gods to have children with mortals. These demigods were usually favored by their divine parent, so those who harmed a demigod could expect consequences. Pilate's fear and motivation to again speak with Jesus is probably rooted in that concern. He's already had Jesus viciously whipped and humiliated (John 19:1–5), though the crowd wants more (John 19:6–7). If there's any reason to think Jesus is connected to some deity, he won't dare have Him killed.

However, Jesus doesn't respond. He knows Pilate has no interest in deep thinking over these issues (John 18:38). He's not familiar with Jewish Scriptures or prophecies (John 18:35). There's no reason for Jesus to debate those nuances when He already knows how this will end (Matthew 20:19). As in His sham trials before the Jewish council, Jesus simply leaves those questions unanswered (Matthew 26:62–63). He only replies when it's necessary (Matthew 26:63–66), and this is not one of those times.

In frustration, Pilate will fall back to the tactics of a petty tyrant: trying to intimidate Jesus into an answer (John 19:10).