The Portal Series is technically one of the greatest games I've played because of the thrilling twist of the whole storyline within the game itself. GLaDos became addicted to test subject that she thought all of them will play by her rules. As Chell woke up (in Portal 1) she tried to kill Chell using the turrets. Chell destroyed her to escape and take the cake (which is at the last part). In Portal 2, GLaDos was reactivated by her and GLaDos immediately placed her on testing again. As you progress, you will see how much destruction Chell (which is by the way your character) did as she destroyed GLaDos. But even though you have destroyed GLaDos, she still helped you after all the things you have done to her (seriously, putting in her a potato?). She let you free with a free-of-charge Companion Cube. And she also gave you a very beautiful orchestra of turrets as you exit the facility.