How does Buddhism deal with psychopaths & serial killer What do the suttas say about such people? Surely they're acting on their ignorance (dependent origination), but can't they make use of restraint/self control?

Is there no penalization in such cases according to the Dhamma? If murderers show no regret, then they aren't even born in hell right now.
By letting law enforcement do their thing?

Regret is not needed for the consequences of your actions to happen.