While everyone has mentioned how Ethan has always been super loyal to Ian, I'm surprised nobody has pointed out that Ian and Anisa's betrayal is made even worse by the fact that Ethan has consistently been a backstabber and double-faced with almost everyone, with Ian being one of the very few exceptions. He literally backstabbed JonTron, who, by his own admission, helped make his career and supported him when he was a nobody.
Ethan has also shown a pattern of instantly disavowing or cutting ties with people when it's convenient for him. For example, he quickly distanced himself from Jordan Peterson, despite previously praising him and hosting him on his podcast. Similarly, he severed ties with Justin Roiland as soon as allegations against him surfaced.
However, Ethan stood up for Ian even since he fell in disgrace. I loathe to admit it, but Ethan has every right to be angry with the BPD-cuck duo.