I would guess that Shatner's behavior comes down more to insecurity than ego. My friend worked at a convention he was speaking at back in the late 80's, around the time of Trek 5. She was quite surprised to hear she would be working as one of the assistants while he was there. She was really put off by his treatment of the help, very dismissive and rude, as if they were of lesser value because they were 'only' assistants. There were several shouts of "don't you know who I am, I'm William Shatner" thrown in for good measure.
To be fair, the Trek 5 period seemed to be the height of his pomposity.