I find devils fans general attitude towards this whole situation to be absolutely hilarious
Ehh the perceived anger comes from the fact that he had the Devils excited for so long. He has the speed and a few other skills where you watch him and say, man the second he figures out how to play hockey he is going to be amazing. And he would for multiple games or months and then seemingly forget how to play again. The skills were always there for some reason he just didn't use them. Other than bitter fans I don't think anyone expected him to go to another team and not thrive, he's that type of player who just needs a kick.
I still remember when he got demoted to the 3rd or 4th line and that line gelled right from the rip, he scored and I've never seen someone look do disappointed scoring a goal. Like he knew, "great, this line works and now I'm stuck on it." That was him in a nutshell. But if he does what's asked of him, he's great. When he knows his role that is. Sometimes he tried to do and be too much and it ended up worse than just playing basic hockey.
Wish him the best, always enjoyed watching him play for the Devils but it just seem like he got stale and stalled out, even regressed a bit. If the move was the kick he needed to get back to and stay at his best form, power to him.
And the Ex comparison, it's more like asking your partner to get a job for years, the second you kick them to the curb, they end up getting a super successful job. It's like, I'm happy for ya, but that's literally all I've been asking for from you for the past 5 years.