Adam had found himself in deadly situations more than once, several times even before he became a Phantom. The slums were far from the safest place to be. In some ways, he was used to fighting death to stay alive and get stronger.
But... Silvana was in this position for the first time. Adam was ready to save her in whatever situation she was in, but now... there was nothing he could do.
No matter how fast Adam could be, Magnus or Nessa would surely attack and take Silvana's life.
Adam had only one way to save Silvana - listen to Magnus and don't make any sudden moves. Adam did not doubt that Magnus could slit Silvana's throat with his scalpel, but what about Nessa?
Well, Adam was experienced enough to realize from Nessa's look that she would actually shoot. If Magnus was ready to kill, so was she.