I dunno... I think he was justified
His "tantrum" as you called it, I thought was justified. When he went back and toned it down I was kind of disappointed.
It's like this... he's providing this comic for free... in his spare time... to anyone that wants to see it.
Then this guy e-mails him... tells him he doesn't update enough, gets on his case about how he doesn't like the characters, and then tells him how to manage it.. "wouldn't it make more sense to work on the comic whenever you have free time... rather than the night before?" (this quote is from memory... so it's probably not accurate :P ). It's more like polite hate mail to me. I would've reacted the same way.
And like Brian said... if that guy doesn't like the comic... he doesn't have to read it.
Anyway, critisism is one thing... but that e-mail was extreme. That guy had what was coming to him.