Police forces are disappointing. Sure, many in the police force are good at their jobs but as many good people, there are just as many dirty cops, taking bribes an looking the other way. That’s how many serial killers got away with what they did, like Sobhraj.
It wasn’t all dirty police though. It was also that forensic science did almost nothing. They couldn’t figure out anything. Most of their evidence has to do with witnesses, who were sometimes faulty. Oftentimes to be honest. If you killed someone then walked away and came back drenched in blood still, you could just say you had been gutting a deer or cow or something and you wouldn’t be caught.
Thankfully, it’s improved but still faulty.