Yes, you know it was bound to come eventually. After all, he is a fan girl favorite. Anyway, I have a LOT to say about him, simply because he is the most textured and human (yes, even more then Chell) character in the game. Where to start? Well, as I played throughout the game I grew quite attached to him (putting myself into the position that during the testing I hadn't had any kind interaction, at all) and came to consider him as a comrade and companion during the grueling trial of escaping. Even so, some of the things he says throughout this first part do reveal that he has a sort of mild contempt and resentment for humans and is a bit selfish. Despite this, his transformation after the core transfer was so sudden and complete that even his mirth starts morphing into sadistic fit of laughter, followed soon after by vengeful transference of Glados into a potato. The fact that he reaches this boiling point after called a moron seems to indicate that he has some sort of inferiority complex perhaps because he knows what he was made for.
Anyway, he's gone for some time after that but when he does reappear, it seems that the air of the game has gotten considerably more disturbing as Wheatley has turned everything about Aperture Laboratories into his own twisted creations including deformed Franken turrets that will walk willfully to their own suicide. Even worst is the fact that once I was put into one of his "tests" he made it very clear that I was performing them merely to give him pleasure and while that in itself is an unsettling concept, it's made worst by the fact that he has a monitor in every chamber to watch me and fumed when he didn't get the response he wanted. The tipping point, however, were the comments he made after he was able to get the desired feeling. One post I read put rather well in that you feel as though you are being violated in some way as you make your way through the tests and this turned the whole idea of completing tests into a disturbing sort of slavery to Wheatley's pleasure. As I moved along with the tests I felt like he is becoming more and more unstable as he seems to becomes hysterical with each new level until, alas, he suddenly becomes cheerfully pleased at the fact that he has a surprise in store for me after I complete a few more runs.
The "surprise" comes early. It's death. Surprise. Apparently he has found two new test subjects that he want to use because apparently he believes that the reason he is not becoming euphoric is that the tests are being done wrong and that the answer to this problem to turn Glados and I into a slice of potato-and-human loaf. The fact that he has no qualms about murdering his partner and friend and now views you as disposable refuse will probably bring tears to your eyes as you grieve for the death of his former self.
If this wasn't enough of a tearjerker, as you hear him yelling at you during the final battle, you feel as though he is slowly losing touch with reality and succumbing into paranoia and even a mental breakdown. His comments aren't snidely quipped but instead are screamed out in a rage that sound riddled with tears. Then as he tries to convey to you just how expendable you are to him by pointing out how many people he attempted to escape with, I realized that he was reacting spitefully because he believed that Chell had used and hurt him. It was so very disturbing and yet quite pitiful to see him reacting in the way a child with a weapon would confront a bully that made their life miserable.
Skipping to the end, it does seem that Wheatley is terribly sorry that things got out of hand. Some may say that his apology wasn't genuine but I disagree because it wouldn't really have done him any good to apologize out loud. After all, no one can hear him or rescue him.
Anyway, Wheatley is adorable but what the creators did with him during the second half of game was pure brilliance. Watching him slowly slide into madness was a terrifying and yet absorbing part of the game and as for Stephan Merchant. The man was brilliant and succeeded in making us fear but at the same time pity his character as he fell apart. I sincerely hope there will be another Portal and that Wheatley will have a role in it.