He's just a murderer who faces no legal consequences. J'onn and Supergirl both know that, yet no action Is taken against him (because he's the showrunners' proxy prosecutor of the pro-immigration issue). This is not like MCU's Bucky while under Hydra's mind control, so one could argue that his taking life was not his fault. No, Manchester Black is simply a mass murderer, and all of tears shed for his girlfriend do not justify that...but again, I'm guessing he will never face any legal consequence for that.
We don't know that Manchester Black will never face consequences. It is still very early. Kara and J'onn only just now learned of his crimes. There are plenty of episodes left for Kara and/or J'onn to confront him. We've seen other villains get killed or arrested for their crimes. It is probable that Kara and J'onn will confront him and stop him. They clearly don't approve of his actions.