I think the use of the word disloyal is being thrown around a lot and it's not correct in this case. The reason why Manly fan's are so annoyed at DCE is with the process of how he has come into the side and won a title, then slowly divided the team that has lead to him leaving after tearing apart the core of a team that was together for over a decade is why Manly fan's are angry.
The former front office are where the blame should start, their constant infighting and not securing Des Hasler as coach and putting him offside was the first step of decline.
The next big one would be Glenn Stewart and how his situation was handled. The team let him go to save cap room to re-sign DCE and Foran. This re-lit the fuse and brought up a lot of anger with the senior playing group who saw DCE and his agent launch a campaign through the media over his first contract which Manly let him out of early and gave him a pay rise from under 100k to about 500k per season.
This put a lot of senior players offside with how it was handled and drove a wedge between DCE and a lot of the others like Watmough, Matai, Lyon, Stewart Brothers, etc.
Last season Glenn Stewart was forced out by the team saving up to give DCE another massive raise and then the flood gates opened. Arthur left to coach at another team, Watmough also left on bad terms and now it seems like Foran and Ballin are probably gone too. Matai wanted out and Brett Stewart did/probably does too.
The drama of the last few months has put a lot of Manly fan's offside with the constant publicity and lack of respect shown to the club by the way Gavin Orr has handled DCE's contract negotiations. To be honest I think there were more DCE stories in the media than there were on actual football stories of the NRL as a whole over the last few months.
The fact he's left the team after p!ssing the Manly fanbase and probably most of the team and front office is why everyone is upset.
And the cherry on the cake (pun intended) to deliver the news right after a humiliating loss is also not going to help you win friends and influence people...
My thoughts are if he's after the money then go and get it...There's no other reason to go to the Gold Coast who are a historically terrible franchise and have nothing to show for it other than millions of dollars lost, the team being moved 3 times already in it's history and a drug scandal that has left them struggling to put a team on the field...So if he wants to leave Manly for that, then good luck to him...
For Manly it's probably better to just bite the bullet and start rebuilding and take a blueprint out of the Panthers who have shown it can be done.
The NRL also looks bad because of this whole system of letting players (and coaches) sign with another team whilst still under contract for their current team. THAT is what makes the sport look so amateur and if it wants to be "professional" and continue to grow then it needs to fix that major problem. I dont know of any other line of business that would let someone do that. So as this season is probably a write-off for Manly before round 1 even started, why bother? It makes the NRL product look bad and it allowed a team that had been the benchmark for the last decade be torn apart from the inside one piece at a time.