Less of a villain decay but also perspective - Arnold was the most iconic part of Terminator and so the writers found a logical way to retcon the actor to continue playing the role.

In universe, John is exposed to the T800 as a hero but his mom only knows him as a killing machine. These characters have to grapple with the role reversal in story. In Saw no one (that I recall) ever said “Wait, isn’t this guy just an asshole?”