...instead of killing them right away?
I mean, they tatoo people's wrists so that they can be scanned, and gather them in concentration camps, etc.
This happens in Genisys and in Terminator Salvation, at least.
It's acceptable that the terminators always waste a lot of time when attacking, but that is due to drama of course.
But the very point of planning a methodical way of gathering people, transporting them, etc. instead of killing them right away seems to me very unplausible.
Note: I know that concentration camps have some reasons to exist if we examine the history of human warfare, but in a machine-vs-human warfare I guess most of those reasons don't apply.