Honestly...(This is a bit long) lol

I honestly think Chang was shocked and jealous about Ed's grandfather being there when Ed got hurt. Like he assumed they were the same type of lonely where they could only depend on each other in their own weird way.

Then he hears that Ed is hurt rushes to the hospital expecting to...I don't know. Maybe be Ed's comfort maybe even solace, help out with what he can because it's only him that Ed has and its only Ed that he has. However, he arrives and there is Ed's grandfather, saying things like "My one and only grandson", and Chang, not that it was anyone's fault, had to share his grandmother with Ed growing up.

I think during that scene he was thinking something along the lines of "So you aren't as alone as me...So you don't need me like I thought you needed me." It felt like to me Chang reflected his needs and loneliness against Ed, and when he realized it wasnt technically the same he became mad at himself that Ed had become something solely important to him, but that it may not be the same in Ed's case.

So to make it so that Ed would be alone and need him like Chang always thought was the case, he affixed the grandfather with the debt he knew would soon become Ed's. Chang's feeling are abit twisted, but I like him as a character. He feels like an antihero sort. The type character where they do bad and you question there methods alot, but somehow in the end they've panned out to be an okay person. Chang feels like that to me.