Blatty implies “then and there” that Karras would have killed everyone in the house he could have killed. He tells the demon, “No! I won’t let you hurt them!” The film shows this when it frames the now-unpossessed Regan framed in Karras’s would-be-strangling hands. Then Karras tells the demon, “NO!” and jumps thru the window. Blatty apparently thought the possessed Karras capable of killing everyone in the house. Blatty has already established Karras’s great physical strength and athleticism, and knows that the reader will look at Karras as doubly-dangerous in a possessed state.
I don’t think a “detailed theory” is supplied by Blatty beyond: “regular Karras = very strong”; “possessed Karras = very strong and very dangerous”.