I think most people have a giant emotional inhibition to admitting to themselves that they're doing something wrong, so they have to lie to themselves that it's justified. I don't. I have no trouble saying "no, I shouldn't do this, but I will". It's just sin. It's perfectly normal.
I'm much more conscious of the possibilities of myself as evil. Most people think they're good and that most people are good and that the few evil people that exist are fundamentally different from good people. At least, they think they could never be an evil person. They can't imagine what it would be like to be evil.
I could turn completely evil. I could just make that decision - or at least, I could've before I met so many other Protagonists. I even came pretty close once, and not under what would've seemed like a huge trial. Besides, it's a spectrum! It's just a matter of how devoted I choose to be.
To some people this makes me untrustworthy, but the upside is that I'm under no delusions about my nature. I have a much healthier mind because I don't have to fill it with lies. Other people need self-deception to sin, and I don't, but that caused other people to master self-deception.