Ugly Mind & Empty Brain
They will be a bad loser when they lose.
There is nothing that insecure person hates more than losing. At anything. And when they do lose, they will tell you all sorts of reasons why their loss isn’t really a loss, or why it wasn’t fair, or why the other person had an imbalanced advantage, or how they know that the other person was cheating. People who don’t define themselves by the times that they lose at things, and people who understand that nobody can win at everything, have no need to do anything but put out their hand and say “good game” when they lose.
Even worse, they will be a bad winner when they win.
Someone who rubs in their win (outside of a playful smack-talk setting), or who tells you every way you went wrong to lose to them, or who goes around telling everyone else about your loss, is an insecure bad winner. Good winners have no need to do anything but put out their hand and say “good game” when they do win.