EDIT:
This achievement seems to be broken at the moment, thanks to commenters for the update. Like the "The New Boss" achievement, it appears this achievement is somewhat fixed, but may also require you to do a fresh playthrough of the game in order to unlock it.
There are a total of 18 Racket Bosses in the game. With the exception of 2 of them, you will get the choice of either killing them or recruiting them. You need to KILL all of them. The best way to make this achievement go as smooth as possible is to NOT wiretap the areas the Racket Bosses are in - this will make Lincoln kill them automatically, and you won't get the choice of recruiting them.
Note that for the second Racket Boss encountered in the game, there's a mandatory setup of a wiretap, so be careful with this one and make sure to press
to kill him.
If you feel that you're missing the benefits of wiretapping you can upgrade Vito to get the "Inside Man", which works much in the same way as a wiretap (It reveals enemies on the map and through walls).