The speaker begins with a bold, slightly defiant claim: fire is their flag, and the reason is that nothing matters. A flag usually stands for a country or a cause, so choosing fire suggests a person with no loyalties left, only destruction and heat. The title, "Arsonism," pushes in the same direction.
But the poem turns quickly. With nothing to follow, the speaker points to "this necessary rose." That is a surprising pairing. A rose is delicate and beautiful, the opposite of fire, yet it is called necessary, as if beauty is what remains when belief has gone. The last lines ask what hands could give more freely, with "unparticular generousness." The word "unparticular" suggests giving without choosing who deserves it, the way a flower offers itself to anyone who passes.
The mood is spare and strangely tender. The speaker sounds disillusioned, but not empty. Meaninglessness seems to leave room for small, open-handed kindness.