Agreed, once he started making threats, I can see the reason for the landing/arrest.
In my experience, after you tell someone they can't do something, you can let them vent their displeasure (yell a bit), but continuing to engage with them only makes it worse. If they had stopped responding to him after they explained the first time why he couldn't use the lavatory, would have had sat down and just grumbled a bit? Sometimes engaging makes it all worse. Threatening to call the police doesn't help deescalate situations (it may totally have been warranted, but I just don't know). *shrugs* Obviously, none of us really know the "right" answer in this case. I don't think anyone except those maybe on the plane know what the best answer should have been, but just some questions to think about. I hope Alaska thinks these same questions. Likely the answer will be the FA did everything correct, but maybe it's something they can be trained to handle better in the future.
To be fair, I have found all AS agents to be generally polite, respectful. Nothing like you get on other airlines. So I doubt the FAs were what I would consider disrespectful. However, that doesn't mean they are all perfect all of the time.