Beatty, a literate man despite his devotion to book burning, sees himself cast as Shakespeare's Brutus in this scene. Right before the lines he quotes:
There is no terror, Cassius, in your threats,
For I am arm'd so strong in honesty,
That they pass by me as the idle wind...
Brutus says to Cassius: "You have done that that you should be sorry for." This reflects Beatty's assessment of Montag, for Beatty thinks Montag has made grave mistakes in embracing books, a destabilizing force, to Beatty's mind, to the society.
Beatty understands Montag as Cassius. As Brutus is contemptuous of Cassius, so Beatty is dismissive of Montag. In lines apropos for this novel because of their fire imagery, Brutus says to Cassius:
O Cassius, you are yokèd with a lamb
That carries anger as the flint bears fire,
Who, much enforcèd, shows a hasty spark
And straight is cold again.
Brutus is saying here that his anger passes quickly. If Beatty had this passage in mind, which comes slightly later in the scene, he too is thinking that he will make it up to the, to his mind, weaker Montag, once their little fight is over.
But Beatty has misread his man: Montag is not Cassius. Brutus survives his encounter with Cassius, but Montag turns his flamethrower on Beatty, killing him. There is indeed truth in Montag's threats.
Beatty's death wish is used to justify Montag' murder and is buttressed by the parallel with Brutus (who also wishes to die). But Montag changes the equation in his ruthless killing of Beatty: perhaps, like Octavius and Antony, he has the qualities that will help him survive.