Why do people jump so easily from "lets build an efficient monitoring system" to terminators?

Probably because humans (especially governments) have repeatedly proven their willingness to take innocent tech and use it nefariously. That's how you go from CCTV being installed to "catch a few crooks" to it being used to monitor citizens for social credit systems. Or people's innocent internet use being hoovered up by massive surveillance projects.

It doesn't seem that crazy for people to wonder at how initially innocent tech might be weaponised.