Many chapters of Space Marines recruit from among the most dangerous criminals they can find as they haev already proved themselves competent fighters. Obviously they dont pick up any Joey Mchivescum but complete and utter pychos. Then they brainwash them to fight for the Emperor.
Case in point Capt. Shrike of the Raven Guard who was a ganger on one of the hive worlds.
Also Paul Verhoven didnt even read all the book. In fact he read the first chapter and thought it was boring. It is Starship Troopers in name alone!