It obviously wasn't a lightly made decision, because either he did something illegal, and thus voided his own contract, or they had to pay him out of it. WGA guidelines are pretty strict on this.
I personally wouldn't worry about the tonal shift being that drastic, a PR script still probably goes through about 40 different departments before it's greenlit for production, so Disney clearly/fully endorsed the "dark" tone and will probably like it to continue throughout the season. Just maybe not as dark as Guzelian made it.