Is Barnabas an idiot? (good question) He knows Julia is under Tom's power, so he stands there and not only shows her the gun he plans to use, but also explains his plan to her in detail. As if she wasn't going to stand in the way.
THEN he turns his back on her and says things like "I see Jennings! He's out there WAITING for you!!!" As if she wasn't going to run out the door.
Or how Barnabas acts so calmly, after Julia has just messed up the plan to kill Tom. He's just like, "well, this was our one chance to do it and it's gone. Better luck next time." If it had been me......
(Barnabas comes out of closet with gun)
Julia: No!! I can't let you kill him!
(Tom disappears)
Barnabas: (slaps his forehead with his hand) Julia!!! What the hell!?!!
Julia: I couldn't let him die!
Barnabas: May I remind you what is at stake here?? Your life, the experiment, Vicki's life, and the lives of practically everyone at Collinwood???
Julia: Well, excuuuuuse me!!! I haven't been getting this much action from a man in a long time---so f*** off!!!