The Terminatrix was created by Skynet for seduction
Really, this is the only good explanation for the Breast Expansion scene.- That could also be a secondary reason for why the 101 models are based off Arnold Schwarzenegger.
If Sarah Connor was in the film...
The script says she would have died the same way is in her nightmares in T2. But for this film, she would Face Death with Dignity instead of screaming in agony. The writers originally intended the T-X to be why SKYNET goes rogue.
Originally SKYNET only turned against humanity because they tried to deactivate it in fear when it became truly sentient, but here it turns against humanity for no apparent reason. Given the T-X is capable of reprogramming machines, and one of her primary objectives is to ensure SKYNET's existence, it wouldn't be a stretch that the T-X was intended to be why SKYNET goes rogue; either reprogramming it to hate humanity, or convincing it with future info. However this plan was cut given it wasn't as important to the overall story/they just didn't see it as necessary. Of course we don't see everything the T-X does and she does reprogram the T-1 drones to do her bidding, so it could still be canon that the T-X is why SKYNET turned rogue. It would also explain how SKYNET in Terminator Salvation knows about events from its future (John Connor becoming its Arch-Enemy, Kyle Reese being Connor's father through time travel); the T-X is from a point where that already happened. The T-850 was set to read-write mode.
Given their normal personality, if it had tried to play on Future John's feelings towards his model number in read-only mode it would've probably fallen flat. As such, Skynet put the T-850 in read-write code so it could convincingly emotionally manipulate an experienced Terminator fighter like John enough to kill him. The downside to this is that the T-850 was emotionally cognizant enough to doubt and be more human. This would also explain why he of all Terminators was picked for the time travel mission; like "Uncle Bob" he was human enough to side with humanity on some level, so Kate Brewster took advantage and reprogrammed him instead of just having him destroyed for terminating the head of the Resistance. While the T-850 appears mostly emotionless like Uncle Bob did as read-only, he's also capable of lying and has knowledge on human psychology unlike the T-800. He was only pretending to be (as) robotic so Present John doesn't get attached and he can achieve his real mission.