[He doesn't need to know Dexter all that well to know this is desperately unusual; he knows how the younger man works, through study and instinct alike. They are methodical. Neat. Not like this-- and of course, circumstances make a difference, but there's more to it than that.
If he thought it had anything to do with him, Brian might be touched.
As it is, he notes the raw, raging twists and stabs as something to ponder later, and takes advantage of having an ally. The raptor turns its focus on Dexter, and Brian pulls away from the flailing claw. He can't get in close enough to eviscerate it without risking himself, so he steadies himself, wraps both hands around the handle of his knife, and thrusts the blade into the dinosaur's eye while it's distracted by Dexter's attacks.]