why do guys speak bad about you to their new girlfriend?after relationships?

Answer 1

Because he is very insecure. And he doesn't want her to think he still likes you. Did you break it off with him or did he break up with you. If you broke it off with him that's one reason, male pride, were all guilty of that sometimes. Personally I would not do this because if the other girl was smart she would be able to read between the lines. He should never bad mouth you that only makes him look bad.

Answer 2

depends.
how did it end? if you ended it usually it's because they're jealous and didn't want it to be over...
if he ended it, who knows.. maybe because he knows he was wrong.