It probably will be before I'm through

[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.

[David L.]

He isn't going to 'try' to persuade him, coax a confession out of him - he's going to extract one; and although he is an 'officer of the law' the ways and means he is going to use to 'persuade' him to confess, well, some of them with be tantamount to physical assault ie against the law!

[imchongjun]

I see. Thank you very much. Now it is very clear!!