He hated everything (and I mean everything) that had been done to the character since Kirby stopped writing him. That's not an exaggeration.
This is going to take several posts, so bear with me as I find pages and whatnot.
The biggest and most important thing is that he turned Etrigan and Jason Blood into one single entity again. Believe it or not, that is how it was back when Kirby created the character. There was no human being named Jason around when Etrigan was summoned. Merlin literally transformed Etrigan into a human form, and he lost all his memories as a result. He thought he was a human, abet one who was seemingly immortal.
Time passes, and writers make changes. After CoIE, Matt Wagner wrote a miniseries that explored Etrigan's history, and his odd relationship to Jason Blood. Surprise, Jason was a human who was bonded to Etrigan! That became the basis of a 58 issue ongoing series by Alan Grant and Garth Ennis. Them being two individuals led to a lot of stories where they were in conflict with each other.
Well, Bryne didn't like this, and finished his Wondy run with an arc that established 100% that Jason and Etrigan are the same being and that the last decade of stories were fake. Etrigan got fake memories that MADE him think he was human, and had accidentally formed a human soul that took shape as "Jason." That was the biggest and main thing that he did, but he was sure to salt the earth from previous runs too.