Why did Prince Brat want to jump into the tunnel?

Prince Brat actually didn't want to jump into the tunnel. Jemmy was determined to escape the outlaws, but Prince Brat was afraid to jump.... he was most afraid of the rats that woul be waiting for them.

The arched sewer stood tall enough for a horse and rider. Jemmy leaped the mud and was in.
But the prince balked. "It's black as night in there!"
"Jump! Quick!"
The prince steeled himself and made the leap. Jemmy advanced into the tunnel, but the prince held back.
"Follow me! We'll be lucky if they didn't catch sight of us!"
The prince stood terrified of the darkness ahead. He had turned dead white.
Jemmy made a grab and yanked the prince after him. "You'll get me caught!"
"I'm--I'm scared, Jemmy!"
"Don't fret about the dark! There are rats in here. Even grown men are scared of em! Hang on to me."