Trashing an ex-friend tells your new friends exactly what to expect from you.

People listen to how you talk about absent friends and apply it to themselves. Running down somebody you were once close to does not read as a verdict on them, it reads as a preview, because everyone in the room can do the arithmetic. Even a fair complaint gets heard that way. The audience learns about you rather than about the ex-friend.

When something ended badly, keep your version short and unspecific: say it did not work out and stop there. Give the detail to one person rather than a group. Restraint reads as safety, and safety is why people get close to you.