In "The Cold Equations" Barton waits until last possible minute to jettison Marilyn because.....
....he can't really believe that he is going to have to do what he knows he has to do. When he first discovers that there is a stowaway on board, he sets out to deal with the problem quickly and efficiently. However, when he realises that it is a young girl, who, what is more, is completely unaware of her necessary fate and the seriousness of her actions, he is completely torn by the inevitable. Note what it says when Marilyn is first discovered:
He stared without speaking, his hand dropping away from the blaster, and acceptance of what he saw coming like a heavy and unexpected physical blow.
He deliberately delays telling her the truth and even calls his superior officer to see what can be done, even though he knows that it is a "vain hope." He says he will not place her in the air lock until he has exhausted all possibilities. However, when he has received his expected answer, he does what he can to help Marilyn accept her fate by letting her speak to her brother and giving her a bit of time and space to accept the inevitable. At the end of the day, we need to accept that Barton's decision to let her live until the last possible moment reveals more about his compassion than anything else.