--trying to apply logic to Terminator time travel is futile. Think of the whole basis of the movies. Humans are about to penetrate Skynet, so Skynet sends a terminator back to kill Sarah. Humans get in, and send back Kyle. That failed, so Skynet sent back the T-1000. But, if humans were there, how did the t-1000 get sent back? Also, wouldn't the fact that John Connor existed mean that the t-800 failed? So why bother sending him back?