I think dialogue is badass when it comes from a badass. Some badasses like to go into detail and some like to understate themselves. Here we see an exchange betweeen those two kinds:
Philip II of Macedon: "If I enter Laconia, I will level Sparta to the ground."
The Spartans: "If."
Some badasses lean toward the serious:
Jeanne de Clisson: "Tell the King of France that the Lioness of Brittany is coming for him."
And some display a certain amount of humor:
Chesty Puller: "We've been looking for the enemy for some time now. We've finally found him. We're surrounded. That simplifies things."
What these people have in common is utter confidence in their own abilities. They know they aren't making empty threats. And other people know it too.
Genghis Khan: "I am the flail of god. Had you not created great sins, God would not have sent a punishment like me upon you."
So I guess you should focus not on making badass dialogue, but on making badass characters.
The Goddess Ishtar: "If thou openest not the gate to let me enter, I will break the door, I will wrench the lock, I will smash the door-posts, I will force the doors. I will bring up the dead to eat the living. And the dead will outnumber the living."