What kind of incompetence? I can't fault law enforcement back in the day for missing on some killers because they lacked the capabilities to process DNA
It was hardly anything to do with a lack of DNA.
Before the three strikes law came into place it was not uncommon for a person to be:
Arrested two times for assaulting a random person
Arrested for attempted kidnapping
Arrested for for an actual kidnapping
Arrested for another kidnapping
Accused of rape - maybe even did time for it
Accused of murder - maybe even did time for it
Let out after 10 years
Charged with kidnapping again
Charged with stabbing some random person
Accused of rape again
Arrested for assault again
etc etc...
Then when they were arrested for the final time, only after it being found that they killed 12 people since the last time they were released, law enforcement could offer nothing more than "whoops".
Serial killers can blend into society, yes. But it would be a misconception to think that they all do. Many of them were lifelong criminals who should've never been released back into the general public. Ted Bundy killed like 2 or 3 people after escaping, but others killed dozens after they were legally let out.