User blog comment:B214/Talk About What You Want! 2/@comment-24098614-20140608224506/@comment-1359322-20140608233524

Then Issei would just be killed off as a Stray Devil much like Kiba would face if he decides to disobeyed Rias' order and pursuit revenge. Devils that don't listen to orders might pose a threat after all. Like Viser who left her master and started devouring humans. That's why you can see many Devils being forced to serve their masters, because they don't want to be killed.