Sadist and torturer Tarn might be, but he sure doesn't seem unafraid of something simple as a knife -- something that Wash is quite aware of, and isn't afraid to use. He lets him try to shift away from the blade, but his arm is firm around him, keeping him trapped against him.

"You don't sound much like someone who's afraid for his life, Tarn. You know that human bodies are real fragile, right? 'Cause I'm trained to know just how fragile." A slight turn of the knife, just to prove his point. "I know you don't know much, can't be too bright if you decided to come back here again. Tell me what you do know. About the base, about the camp, too."

Even if none of it will be useful, he wants to hear it all, and it seems like your best bet, Tarn, might just be to try and distract him enough with something to get away. Wash does seem intent, and while he personally is quite ready to kill him, he is holding himself back from it, here.