Psychology and reasoning behind bad-mouthing an ex? How to respond?

I don't know, to be honest, but I do have some theories.

Here is one: I think that when people are together with someone, they feel the need to pretend to be happy to people on the outside, even if they aren't, because they are scared of what other people think. But when they then split up with the person, the rush of relief that they can finally stop pretending is so great that they temporarily lose common sense and just take great joy in badmouthing said person.

There's a psychological theory of relationship breakdown that talks about phases you go through, particularly relevant to this thread are the social phase and the grave dressing phase whereby each partner creates the story that will be presented to their social circles. The stories for each partner are rarely the same, because they're constructed to make the person feel better about the breakdown of the relationship. I think there are loads of reasons to make the other half come across badly, like to make you seem like the winner of the argument in front of your friends, to make it seem like you're too good for them and boost your confidence after the blow of breaking up, to point out to yourself what their flaws were in order to make better choices in a new partner, etc etc.

It all depends on the situation really, some people break up quite well and go through the grave-dressing phase without making the other person out to be a total dick, and instead just justify it by saying that you weren't a good match or the timing wasn't right or w/e.

To be honest it really depends on the breakup. People often say things they don't mean when they're hurt/upset/angry, so if it was a particularly nasty breakup and someone was mistreated, then it's just a way to vent out the emotion. I think both people also feel a degree of bitterness that it didn't work out and so feel the need to justify the reasons for ending the relationship/the relationship being ended by one of them.

When my ex and I split up, he was absolutely horrific towards me (and still is), and he caused a lot of pain and upset, not just for me, but for all our old friends. During the first month I wasn't particularly nice about him because I was really hurt and betrayed but still had really strong feelings for him. He let me down when I needed him most, but since then I have been the bigger person. I apologised for the things I said, but tbh the way he's behaved is much, much worse and his actions speak for themselves.

I think if he had attempted to be civil with me/make an effort with our mutual friends after, instead of ditching them, I would've been ok. It was just the complete lack of consideration that hurt.

You're mourning the loss of what used to be and I guess you go through a grieving process for the relationship.

Sorry to hear of your experience, hope it didn't cause any irreparable damage.

I've forgiven him for it I just hope one day he can forgive me too. It's always a really difficult situation because so many different things can influence peoples' behaviour after breakups. I know that the people my ex hangs around with now play a big part in how he behaves towards me. They actively encourage him to be nasty. Drink and drugs etc also change people if they start doing that kind of stuff to cope with the breakup.

The best way to respond is to just ignore it and rise above it. I tried to talk to my ex about what he and his friends had been saying/doing and the whole situation escalated ridiculously. You're best off just leaving your ex to it, they'll stop eventually when they begin to get bored and realise the only option they have is to get over you and accept that they did wrong/the relationship is over.