WARNING: THIS CONTAINS SPOILERS, IF YOU HAVE NOT BEATEN PORTAL 2, WELL THAT'S SAD THE GAME HAS BEEN OUT FOR 2 YEARS NOW, GET MOVING
Okay here is why, according to Greek Mythology, when some one dies and goes to Elysium (Heaven) they are accompany by their best friend and hear singing, at the end of Portal 2 turrents take aim at chell and then start singing, at the very end the companion cube is thrown out side with Chell, the companion cube is Chell's only friend and by default making the companion cube Chells only friend.
Chell is was shot and then music was played and you can see in the background the companiona cube, their both on there way to Elysium and both dead, GLaDOS has killed her the only answer to defete chell was the simplest. No longer Caroline and now the empty shell that tryed to kill you in the first game "Caroline deleated, good bye Caroline" after the once emotional mother was gone it's now just back to evil GLaDOS, back to the old her trying to kill chell, old habbits die hard "Now thats done. Before you my life was pritty good. All I did was test. I couldnt complain. Nobody tried to murder me. Nobody stuffed me into a potatoe. And then you came along. You monsterous, mute, freak. Being Caroline taught me alot. The solution to a problem is normaly the easiest. And I have to admit, killing you is hard. Now I'm going to let you go...dont come back! at the end you see the turrets deploy and 3 seconds before they start singing, in the game it states that the turrets have 3 seconds before firing, Chell is dead.
Note: The theory that there is a person in the copanion cube, the copanion cube was Chells best friend, it was dead long ago due to extream heat think of it still being alive in the first game and you throw it in fire and the person in it dies, the person has been dead for a while, now it's Chells turn
Note: just to add on, Prometheus and Epimetheus created humanity, but Epimetheus (Wheatley) was useless. To help, Prometheus gave humans ﬁre (GLaDOS gave Chell the Portal gun) and was cast into the pit of Tartaros for it. Signs? Elysium was a heavenly ﬁeld of reeds, and Portal 2 ends in a ﬁeld. The depths of Aperture have 'Tartaros 9' on the walls. And the painting of Caroline and Cave shows Aeschylus(he was a Greek play writer) - writer of Prometheus Bound.