The thing with Chad is...yes, he did a whole pretend tumor thing with Abby to gain sympathy so she would be with him (remember how she could not make up her mind between Cameron & Chad?), but when he went to Boston, and then was with Stefano for a year, Stefano imparted all the wily thinking, how to manage the businesses, how to keep you plans to yourself, how family was so very important, etc. etc. He had a long time to sort of brainwash. But, Chad was still Chad, learned lessons well, especially how to keep things to himself, which he seems to do with his father. They play cat and mouse all the time.
In contrast, EJ was taken under Stefano's grip at an early age, so spent a great many more years getting the brainwashing. We saw EJ do many evil deeds before the rape of Sami, and after, as well. His arguments with himself over being evil vs. being good were small, as though he had good moments, mostly the evil won out.
Chad has really played his cards close to his chest. He does well at having Stefano believe he follows the DiMera path, we were never quite sure where he stood with Kate running DiMera Enterprises after Sami left. Yes, he was angry with EJ for his dalliance with Abby, and though he had dumped Abby for Melanie, he was angry with Abby as well. Or...maybe he was not, just pretended when he felt he had to protect her from whatever Stefano wanted from her, because it was very apparent to Chad that Stefano had an ulterior motive for wanting Chad to be with Abby.
I think Billy Flynn has done a wonderful job portraying the conflict that Chad feels, the hate he feels for the hurt he has caused Abby, which he figures is necessary to protect her. I have grown very fond of character (and I was not, in the past).