As this chapter begins the reader has just learned of Provis's terrible background and also of his involvement with Compeyson.  When Pip states in the second paragraph of chapter 43,

"A new fear had been engendered in my mind by his narrative; or rather his narrative had given form and purpose to the fear that was already there.  If Compeysonwere alive and should discover his (Provis) return, I could hardly doubt the consequences. "

Pip is suddenly aware of the fear Compeyson has of Porvis. Provis had tried to kill Compeyson while they were in prison.  Now that Provis is out of prison Pip is afraid that because of Compeyson's fear of Provis, and if Compeyson is given the chance,he will turn informant against Provis and try to get Provis sent back to prison.