Quote from Empress of August
I think he's neither good nor bad guy. He just acts on his own purpose that is to obtain the Hougyoku and to return something back to Rangiku. At the brief flashback there was a scene in which he witnessed Aizen's lackeys stole something from Rangiku (maybe part of her soul, since souls are the main component of the Hougyoku).
Remember Gin's demeanor towards Rukia before, Gin must knew that Rukia had the Hougyoku all along. When he met Rukia on her way to execution, he actually pondered on rescuing her so that he might have the Hougyoku. Whether it was his dislike to her or his considering that it would be mightily difficult to get the orb out if he do so, it was quite obvious that he chose to get that orb no matter what.
And I think he really dislike Aizen but the way he always has that smile freaks me out.
Remember Gin's demeanor towards Rukia before, Gin must knew that Rukia had the Hougyoku all along. When he met Rukia on her way to execution, he actually pondered on rescuing her so that he might have the Hougyoku. Whether it was his dislike to her or his considering that it would be mightily difficult to get the orb out if he do so, it was quite obvious that he chose to get that orb no matter what.
And I think he really dislike Aizen but the way he always has that smile freaks me out.
The answer is no. He is not a bad guy. The flashback showed that he planned to kill Aizen from before Aizen even knew who Gin was. The thing that Aizen stole from Matsumoto is most likely not something physical. He was probably referring to something like her innocence, or her dignity. He wanted to kill Aizen because Matsumoto was the most beautiful thing he had ever seen (I think he even said something along those lines). When he saw that those people beat up on her, he sought out vengeance, and since he saw Aizen as their leader, Aizen was his target. Everything he did from that point onward was just a means to get closer to him and kill him. Really, when you think about it, he didn't do anything overly evil. He could have killed many innocents at anytime, but he chose not to. He was just putting on an act.