Was William Shatner really a jerk in the 60s, 70s, and 80s?

Question

This answer lists several snippets from articles theorizing why Shatner did not make an appearance during the unveiling of the NASA shuttle Enterprise. Half of them are less than positive and sound like Bill was a known jerk back then. I've only ever heard good things about him and am not sure why all that hate was thrown around.

Here are the excerpts in question (emphasis mine):

But where's is Shatner? He probably didn't attend, because he's too cool for school. Or maybe he was banging a NASA girl in the shuttle bay, who knows. But all is forgiven: At least he went back to NASA to record the last wake up call for Discovery.

If you don’t recognize them, that’s the cast of the original Star Trek television series, minus William Shatner who was apparently too important to show up. (Actually, I don’t know why Shatner wasn’t there, but it’s no secret that he frequently behaved like a colossal jerk during the ’70s and ’80s.)

Therin of Andor: Shatner made a habit of being absent from such high profile events, probably his agent holding out for an appearance fee. He was also absent from all the other actors' Hollywood Star Ceremonies (except Nimoy's and Roddenberry's), and more recently George Takei's wedding.
WHF: He was probably the only one with a job at the time and had to work.

So was William Shatner as bad as these people say he was?

Answer 1

Signs point to yes.

Just about every castmember of Trek has a story or two about Shatner that cast him in a very poor light.

Here's a legendary recording of Shatner recording his lines for a voiceover, and treating the technician and voice director like utter garbage. Condescending and demeaning -

He's made great attempts to act better in recent years, and at least try to explain past behavior. But George Takei still won't talk to him (and Shatner says it's all on George) and there's still plenty of people who won't buy his "I was booked" explanation for missing Nimoy's funeral.

Additional - quote from a Newsweek interview

“I had never really got to know [Takei],” Shatner writes of their frosty relationship. “He would come in every so often during the week while we were shooting Star Trek. I was busy learning lines and dealing with my life, so I really can't remember a meaningful conversation—I'm sure that would be my fault… my lack of attention.

“Nevertheless, when we all wrapped that last day of shooting it was all meaningful for all of us—Star Trek was cancelled. Until this moment in his apartment we had not spoken. Not so long after that very friendly time he began to say very mean things about me. Why?”