Is the D.A. really so dense that he has to ask how Calahan knows Scorpio will kill again? He's a serial killer you Idiot! Serial killers always kill again! Moron!
Quote: I don't like repeat offenders. I like dead offenders. Ted Nugent.
I think at the time, 1971, serial killers weren't as common as they would become as that decade and the '80's played out. Also, law enforcement agencies were remarkably bad at sharing information with one another. That was how Ted Bundy got away with so many murders in multiple states over so many years. After Bundy, local law enforcement agencies started doing a better job of sharing information. Also, the FBI got involved in helping state police forces understand the serial killer's mentality. This was the core theme of the miniseries Mindhunter.
So the D.A.'s aloofness could be the common understanding (or misunderstanding) at that time. Really shows how Dirty Harry (both the character and the movie) were ahead of their time.