My best friend broke up with his girlfriend and I'm going to message her.
Do you have a particular reason for messaging her?
I want to make my best friend feel like a loser and an idiot for losing someone so pretty.
Are they really your best friend if you are purposely trying to make him feel negatively? You shouldn't be trying to get with your friend's exes in the first place, but this is your best friend so this is even worse.
He's one of those beta males that gets cheated on. I can't help but complain about the situation and feel it's natural for me to message her.
Trying to date your friend's ex is inappropriate and involving yourself in his relationships generally speaking is out of line. I think you should let your friend handle his own relationship style and you should focus on meeting someone different.