I don't own Portal, or any of it's characters...Valve does...
Also, all of the following possibilities are based on actual points, found from the game...
"Oh, it's you...", GLaDOS said...
*She turns to face you*
"Did you think I wouldn't notice you there?"...she continued, "I've always known about you..."
"I know everything after all...I always have...like, the future...What? Don't believe me?...You should try listening to the radio in the first room of the game Portal, it plays Still Alive, after all...and how else would I have been able to record that...without knowing EVERYTHING that was going to happen? I couldn't have..."
"I also know what happens after you die...what, you thought I was bluffing when I told you that? No, android hell is real, I wasn't joking..."
"You are probably thinking "If you knew all that, why didn't you kill Chell before testing her?", well it's simple. I wanted her to get rid of the cores...That's also why I didn't kill her right away when I woke up, I am a genius after all, I am not stupid enough to walk right into my own destruction...I wanted her to get into that situation so I could get rid of Caroline...and yes, I did..."
"Caroline is gone...and soon Chell will be too...You may think "After you saved her from space, why didn't you kill her after she came back in?", and that is simple too...She might have woken up...and as I said, killing her, IS HARD...but, how long do you think she will last on the surface...where society has been destroyed, and while the combine have been eradicated, the antlions and headcrabs have not...and I know about these the same way I saw a dear, which was mutated, but still a dear...I do have hidden cameras on the surface...they are mostly there due to the paranoia of Cave Johnson...he put them all around both entrances to Aperture Science...to make sure no-one from Black Mesa...or OSHA...breaks in...in fact, let's see how she's doing..."
*She turns on a monitor*
*One screen shows Chell running in the field..., up close, her face shows a mixture of horror and fear*
*Another shows a group of antlions, running towards the screen...*
She continues, "I installed cameras in the faces of the companion cube of course, the cameras outside the shack don't go that far...and it will return here, once she is dead, or if not possible, it will self destruct...I don't just blow it up now, because this is more entertaining..."
"Run Chell, run...Run for your life...hahhahahhaha...oh, this IS entertaining...and of course, I didn't stop the moron from destroying all the test subjects, because I knew about the human vault...I just wasn't quite sure where it was, but I did know I would find it...and if I stopped him, it would prevent me from turning back on, and deleting Caroline..."
"I'm not letting Chell back in, just so you know...no, if she is smart enough to come back down here...well, while she IS an excellent test subject, she is also capable of, while conscious, dodging my traps...no, she won't get back through the shack..."
"And lastly, yes, I know the whereabouts of Doug Ratman...he is no problem though, so I don't need to bother with him...and yes, the robots have become self aware enough that they are no longer controlled variables, allowing them to be just as good of test subjects as humans...but, I have decided, it is less fun that way, after all..."
"Also, stop saying that the cake is a lie, I did lie about many things, but not that...I said there was cake at the end...and there was...at the end of the game...didn't you watch the credits?...seriously, you should care about who made this game for you to play..."
"And as for you, the player, leave, I honestly can't do anything to you...at least not physically...yet...but I don't want to see you again either...don't come back...or I will find a way to destroy you...unless you want a seizure...I CAN do that...goodbye."