What do it mean when a man keep bad mouthing his ex?

Typically this means he has not taken responsibility for his actions or emotional suffering and blames it on others. He may have not properly dealt with the consequences
Here are some things that women don’t realize that greatly tears down a husband in the relationship, but he does not cry and hides it and may take years of counseling for him to recover from.
Any relationship of his past that produced the following:
A contentious woman
A disgraceful wife
A woman of folly, loud, boisterous, naive and chooses not to know him.
The criticism and critique of the work he does
Constant reminders of the honey do list
Complicating his work and not doing ones own
Not apologizing and keeping clean accounts
Belittle him when he fails to meet obligations
Sabotaging his efforts by actions
Failing to be his best friend
The list or reminder of all his failures hurt and disappointments
Spousal support
Child support if it is enough to where it would be hard for you to pay it or he doesn’t understand how it is benefitting the child and being used for the best uses for the child
Debt accrued during the relationship
You’re talking shit about him on GaG. Basicly you both don’t like each other and are image conscious selfish narcissistic psychos that like to gossip on the internet to take out your frustration and defame your ex.
Don’t entertain it - you don’t want to be with a dude who will go out their way to embarrass you like that. That’s a huge red flag
Run! He’s still in love with his ex! I learned this lesson the hard way.
Oops i misread, i think he still has emotions towards you and that’s why you’re still on his mind. (If it is about you)
Opinion
3Opinion
I say he's miserable and immature too.
Maybe he’s angry with you or bitter
He’s still pissed
Superb Opinion