Charle: Because Skynet is convinced ALL humans want to destroy it. When the people who created it realized it was THINKING ON ITS OWN (apparently an hour after the fact) they panicked and tried to shut it down! Apparently SN also panicked (or just concluded humans=death) so it launched the nuclear missiles to kill them all off. But when some humans managed to survive, it decided it needed to develop better ways to kill them. Thus the Terminators, robots that looks like humans so they can infiltrate human groups. With John Connor’s help (who apparently knew the future, from what the Ah-nold Terms had told him -in the movies-) humanity not only survived, but managed to defeat SN’s armies. In a final desperate move, SN used experimental Time Travel technology to send Terminators into the past hoping to kill Connors before he ever existed: first a T-100 (The Arnold model) to kill Sarah Connors (as seen in the first T movie); and then the experimental liquid metal Terminator (T-1000) to kill John Connors as a child. I do not know if it sent both Terminators at the same time, or if it sent the first, realized it had failed, and then sent the second one.
As for the GOOD Terminators, these were a couple of T-100 models that Connors captured and REPROGRAMMED to help save his mother (and himself) in the past, as seen in the movies. (Note: I don’t know how the humans got the time machines; maybe they found one, or found the one Skynet used. We don’t know if he sent both Terminators at the same time, or one first and then the second when Skynet sent the T-1000 to the past. Also note that the first thing that Connors sent was HIS FATHER, who died fighting the first evil Terminator (in the first movie) but not before impregnating Sarah. Which begs the question: did John -or his father- knew he HAD to go back to father John? And that he WAS doomed to die?)
What would Skynet have done if those paranoid scientists hadn’t tried to shut it down? Who knows, maybe it would have coexisted peacefully with people, or maybe it would have later decided that we’re a waste or resources and killed us anyway.
What was Skynet planning on doing after all humans were destroyed? Probably create more sentients computers or robots so it wouldn’t be alone. Or maybe, not having been programmed to do anything other than fight wars, it would just lay there.
What Arnold-bot meant was that, if the people he was sent to protect died, he probably would just have sit down to rot. (Or rather, self destruct to avoid anyone from duplicating the Term technology.) However, the T-100 that saved the Connors seemed to be developing emotions…
Please note I haven’t seen T-3 yet, so I don’t know any revelations made in that movie, though I’ve heard from some T-fans that it seems to contradict some things stablished in the first two movies.