Re: Not seen R1 yet but...
"... the bratty bad dude wasn't convincing,..."
I really don't think he was supposed to be convincing; he was supposed to be exactly what he was shown as. A spoilt "brat" with an exaggerated idea of his force ability due to his family heritage and has had *some" force training by somebody that is not fully capable of training force users. This is shown by his "Light Sabre" which very clearly isn't any such thing but is a facsimile of one.
I've read, and heard comments along the lines of "how could Rea beat a Dark Lord of the Sith"? That's rather the point I think; Kylo was not a Dark Lord of The Sith at all, he just thought of himself as such.
I think he was portrayed as a "bratty bad dude" because that is exactly what he was supposed to be and that is how Rea was able to defeat him even while being untrained. She is inherently much more powerful in the force than he.