terminators are not really people but your right in saying they are sentient. What do you expect when someone is being bad you think someone will let them get a way with it. Its the classic battle of good verses evil. There is good and bad in all of us but in some there more good then bad and in other theres more bad then good. But you do get movies here and there with the big bad being centre stage etc.
That's probably going to be a loaded issue that we are going to have to deal with in the not too distant future, the moral issue of machines being sentient slaves vs being non-sentient tools.
But for terminators that doesn't really apply. A terminator is a smart bomb, it's AI is a probability engine programmed with a library of scenario/solution options from which it selects that most effective to accomplish it's goal of killing humans while protecting valuable assets, being Skynet and other terminators. It doesn't contemplate it's own existence and it doesn't create it's killing solutions, everything it does is pre-programmed by Skynet. In the few instances where Terminators are seen defending humans, the assets/targets categories have been flipped.