In Portal 1, before you escape the testing track, GLaDOS never actually attempts to kill Chell. She has no reason to - you're just going through test chambers like you're supposed to, and then (for whatever reason it may be) you're meant to get incinerated at the end. Then you escape, and the facts are that she can't actually do anything about it. She has no control over anything outside the test chambers and her own room, effectively, and once you do get into her room then she tries to kill you, and fails. So there's Portal 1.
In Portal 2, at the beginning, she doesn't actually want to kill you. She wants revenge, and her idea of a good revenge is to test you for the rest of your life. Obviously, that plan gets messed up spectacularly by Wheatley helping you escape, and as the 'Good Listener' achievement shows, GLaDOS is more than happy to kill you at that point by trapping you and pumping the room full of neurotoxin. During your escape, yet again you manage to get far enough 'behind the scenes' of Aperture that she can't do anything about it, and then go on to disable the turrets and her neurotoxin emitters.
GLaDOS would have definitely killed you when you fell into the 'GLaDOS Shutdown and Cake Dispensiary' trap, had the aforementioned turret/neurotoxin disabling not happened. Then the core transfer happens, she's in a potato and then she's completely harmless.
She doesn't kill you at the end because she doesn't want to any more. It's not because 'killing you is hard', not really, because at the end she really could have done whatever she liked - Chell was unconscious for however long, and GLaDOS did absolutely nothing because she didn't actually want to.
Also, about her creators - it's safe to assume that they died during her activation, although obviously some people did escape, so maybe some of the people behind her creation survived, but it's unlikely. She wasn't made to be as vicious as she is, no. She was just made to run the facility, but because of all the cores they attached to her (which drove her a bit insane, basically) and the fact that Caroline never wanted to become GLaDOS in the first place, she was really angry when she was activated and just took it out on everyone there.
Hope that's helped answer your questions!
Leon