you do have a point, it is a little confusing about when Gamora had a change of heart...I was always under the impression that she changed sides because of her relationship with the other Guardians...but she was already having second thoughts about her mission and following Thanos even before she grew close to the other Guardians. I wish the first movie had clarified this issue about exactly when Gamora was starting to turn against Thanos...maybe at the beginning of the movie Gamora had already had enough and upon meeting the other Guardians, they gave her all the more reason to change sides, they were really her escape from Thanos to a better life.