I think there's also the simple matter of Ken's ego and pride over not wanting to ever let it be publicly known that he did no actual writing. It was obvious that was a sore subject with him and that his desire to murder Jim was ultimately rooted in the fact that sooner or later he'd be exposed as someone who never had any real creative talent. With an egotist like Jack Cassidy in the role, it's very easy to believe that pride could motivate a guy like that to commit murder and try to get away with it using the one brilliant creative idea he ever really had (I wish the final episode as aired had made the point that I think the paperback novelization did that the reason why Jim never used Ken's alibi scheme for a Mrs. Melville novel was because it was too good. That he couldn't think of a way Mrs. Melville could break it).

In seeing how Ken and Jim are such polar opposites in personality and lifestyle, it could be speculated that Ken's motive in murdering Jim, besides jealousy and "the ego", as you succinctly put it, "Bryce", was driven by an anger of having had to work with someone for so many years with whom he seemed to share little in common. There's a sense that Ken has distinct personal pleasure in killing Jim, such as when he devilishly raises his glass to "toast" Jim's corpse, or the perpetual smirk he has the whole time he's manipulating Jim's emotions and naivety in preparation for the murder, as though his killing Jim not only satisfies his greed and ego, but also a desire to be relieved of a friendship that had become tiresomely incompatible (that is, in the sense of Ken having to not only maintain the lucrative sham of being a writer, but the far less enjoyable sham of working with and relating to someone wholly devoid of his own ambitions and "values"). Of course, there's nothing explicitly stated or shown within the narrative that indicates Ken was upset with Jim before Jim's decision to write on his own, but again, given their drastically different personalities, I can't imagine that Ken would ever genuinely be interested in a friendship/partnership with Jim, other than what it affords him in fame and wealth.
One of my favorite scenes in the episode is where Ken expresses his contempt for Jim very openly - "Whoever thought that idiot would write it down!" I love how the line leading up to that ("I must have told it to Jim over ... years ago") is said so sentimentally in a way, with the accompanying music eerie and reflective, almost in a seeming attempt to make the viewer think Ken is finally going to show remorse and pity, only to have both he and the music resume their discordant ways.