"You can't kill Nicodemus."

I got to thinking about the line by Sullivan, like most people I first took it to mean Jenner couldn't kill Nicodemus for obvious moral reasons. But then again, the remark could also have been a kind of foreshadowing, suggesting that Nicodemus would be immortal, and indeed he did appear in the Stone just before Mrs. Brisby was about to use it.

Who knows, Sullivan may have had a lot more respect for Nicodemus than Jenner clearly did, and immortal is what he meant by that line.

More likely, the very idea of killing one of their own was really shocking. Till that point, Jenner had simply been Nicodemus' political opponent, after all. Nobody apparently expected him to be insane enough to kill someone.

Also, the entire rat community probably had a lot of respect for Nicodemus. Possibly Sullivan couldn't imagine how they'd survive without his leadership.

Since I was used to the book when I saw this scene, I was shocked that Nicodemus was killed off, and sort of sad. :(