Who are "they"?

I was thinking about the scene in TSON when Jenner is explaining his plan to Sullivan, and Sullivan disputes the idea of killing Nicodemus, and Jenner says
"No taste for blood...they've taken the animal out of you." Now, it's logical to assume he means the human scientists at NIMH, but might he also mean the other rats who wish to move to Thorn Valley? In other words, perhaps Sullivan had been listening to Nicodemus and others in support of the Plan and Jenner felt that had made him "soft".

I have to admit that I myself never thought of this angle before. It's a very good possibility.

That is a reasonable statement, but you have overlooked a single detail. If Sullivan had been listening to Nicodemus, then why did he still hang out with Jenner who led the opposition to the Plan? Also, if Jenner knew he listened to Nicodemus, then why did he have him as his accomplice for the attempt on Nicodemus' life? And why didn't he try to warn Nicodemus when he had the chance?

To answer those questions, I would suggest it boils down to fear. Sullivan may have stayed with Jenner out of fear of him, and also would not tell Nicodemus, again because of the fear of what Jenner might do to him. We know from later events in the film that Jenner had no qualms about turning on Sullivan.

As for why Jenner still trusted Sullivan, that may come down to his believing that Sullivan would not go against him, again because he was afraid of Jenner.