I think it's a little silly that in a book that's supposed to be fairly down to earth and realistic Frank has NEVER killed an innocent civilian (are we sure he hasn't? I vaguely recall him angsting about something like that at one time or another, though I could be thinking of something else). Trained professionals have been known to clip some poor sap who was just in the wrong place at the wrong time, I think it would be fair to assume the same thing might happen to Frank on occasion. Personally I think Frank brutally murdering an uncover police officer or something would make for a fairly compelling story. Even someone like Frank makes mistakes, and when you routinely use deadly force those mistakes are likely to have deeply regrettable consequences. It's a risk you take when your form of justice is irreversible.