I can see some being resentful for leaving before the job was finished, but the hate and trying to act like he can't coach is kind of extreme.
You laid out the reasons for a coach leaving with stress on the highlighted part. I don't know if he has more security. I've said before and will continue to say, in our current state our coaches have a 3-5 year shelf life unless someone just really likes the environment here and is happy making a little less money with almost no shot at a title