A lot of you seem to be questioning Edward's bafflement over Aro's treatment of Bella. I'm not sure I would classify him as being 'slow', but he is perplexed.
First, Aro is older than he and is considered a respected Dominant in the community. As that, it would be assumed that he could treat his 'property' with some level of care. He does know that she was there against her will, but again he doesn't know if that has always been the case. Edward has no idea if there was an arrangement/contract/etc. He is still in the dark on this rememeber.
Edward is good man that helps people on a daily basis. It is what he does, who he is. He is a Dominant, but that means that things are consensual. Just the reality that what happened with Isabella wasn't is hard enough for him to swallow.
Everytime he founds out some new horrible thing that she has been through it hits him hard. Some of that is because of who he is, but some of it is because of who she is. As has hopefully became obvious, he is falling in love with her.
Part of him is trying to give Aro the benifit of the doubt, but he is realizing that he just can't. Everything he believes about being a Dominant, which is what he has always viewed Aro as, is pressing against him in every way possible. His instincts are to control, yes, but also protect. Aro seems to only have one of those traits, and it is hitting him very hard.
Another thing to remember is that Edward is only 24. He is still considered a young Dom even though he has been doing this for five years.
Hopefully this explains a few things for everyone.