It's never explicitly stated as far as I can recall, but Quentin Tarantino has always been very deliberate in his character development. Very often when a character does or says something, it is subtly explained in another scene. Along that trope, these two occurrences are definitely the "key" to understanding each happening.
The story about killing the general's son started off subtle and tame and then escalated each time the general didn't react, making the story seem intentionally taunting.
Almost directly after that scene, it is revealed that Warren's Lincoln letter is fake, revealing that Warren is willing to lie and deceive both to make friends and to make enemies.
We know this character is a survivalist. He'll make up stories to get in the favor of whoever he can, whether it be to get into a carriage and out of the cold, or to become allies in unfavorable circumstances. He'll also make up stories to goad his enemies in to drawing weapons with him.
There was another moment where I sensed Warren was being somewhat dishonest to his advantage. When the prisoner was propositioning Sheriff Mannix, and threatening him with "15 killers", Mannix openly announced that he didn't believe there were 15 more gang members waiting for him. At this time, Warren laughed manically, which kind of makes me think (being a bounty hunter) he knew that there were in fact 15 more gang members, but allowed Mannix to think it was a lie so that he would retain him as an ally.