[imchongjun]

Hello, teachers.
Would you please tell me what the last line of the following passage means?

(Doan, a detective, is talking to a girl about a criminal)
"You mean he (=criminal) wants to come back and give himself up and answer for his crimes?"
"Yeah."
"And you're going to try to persuade him not to?"
"Not try," Doan corrected. "I am going to persuade him."
"But that's wrong! That's against the law!"
"It probably will be before I'm through," Doan admitted casually.

I guess "before I'm through" means "before I will have finished persuation", but I don't understand what the whole sentence means.