MTG Salvation

I was playing 3-player EDH on cockatrice and countered a Rhys the Redeemed player's Beastmaster Ascension because he was ahead (he had t1 Exploration, so he had way more mana than either of the other two players). The Rhys player then kicked me out of the game. When I sent him a PM to ask him why, he responded "Because I don't like people who don't let me do what I want easy." Is this normal or reasonable behavior in an EDH game?

That's not reasonable behavior anywhere. Can you imagine a sports player crying foul because the other players won't let him/her do what (s)he wants easy?

That said, EDH has this weird (to me) mentality where each player should strive to make the game as fun as possible for everyone. Which naturally begs the question, what is defined as fun?

This is made all the harder because it's Cockatrice and you likely never met the person IRL. People online tend to be far more immature than in real life because all the norms fall away. All I can say is, find another player and just blacklist him.

Trice / MTGO / Online commander in general is a terrible thing because there is no social value to the format. People do really jackwagon stuff because there is little to no consequence in doing so. If you were to throw a tantrem like that IRL ppl would just not play with you but in online commander you really dont have that same thing. I have long since given up on playing online magic because it just seriously lacks the same elements that make the game fun in person.

So, yes your experience is about normal par for online commander. Unfortunately its not very good....