Roman is fine right now. The only difference between him and a full fledged heel right now is booking. He's still beating bad guys. That's it. Everything else he does is at best gray and honestly leans closer to the heel side.
Baby faces don't come out declare themselves #1 contender the night after they lose. They don't give attitude to legends like The Undertaker. They don't get in the GM's face and demand things. They don't commit vehicular manslaughter. They don't tell the crowd how much they don't care about their opinions. They don't get cocky and talk down to guys like Joe.
I mean, if you want to be a mark and insist that he needs to """turn""" in an obvious way, like beating up Balor with a chair or something, then go ahead and bee a mark. But the evolution of Reigns since the Rumble has been obvious to anyone paying attention. People are just booing and hating him out of habit at this point like they do Cena.