"Soliloquy of the Spanish Cloister" begins with one of the most famous moments of onomatopoeia in English poetry: a guttural "Gr-r-r." This growl (and Browning's fondness for onomatopoeia in general) displeased some Victorian literary types, who saw such noise-making as brutish and unpoetic. That, of course, is exactly the point here. By starting this poem with a growl, Browning suggests that his speaker—a monk in a Spanish monastery—is more a beast than a holy man.
The speaker's outburst is directed at one "Brother Lawrence," a fellow monk he watches from the shadow of the cloister (a sheltered walkway surrounding the garden at the heart of the monastery). Brother Lawrence doesn't seem to be doing anything so very objectionable: he's tending flowers, taking gentle care of his roses and myrtles. But the speaker registers his every move with venomous sarcasm:
What? your myrtle-bush wants trimming?
Oh, that rose has prior claims—
Needs its leaden vase filled brimming?
Hell dry you up with its flames!
The speaker, these lines suggest, hates every single thing Brother Lawrence does; his most innocuous action is reason enough for the speaker to damn him. This disproportionate hatred tells readers a lot more about the speaker than it tells them about Brother Lawrence. As is so often the case in Robert Browning's dramatic monologues, the speaker here will unwittingly tell on himself, revealing his own pettiness, nastiness, and self-deception with every word he says.
The poem's percussive trochaic tetrameter—that is, lines of four trochees, metrical feet with a DUM-da rhythm—feels as insistent as the speaker's hatred. Listen to the rhythm of the stanza's last two lines:
Needs its | lead en | vase filled | brimming?
Hell dry | you up | with its | flames!
Notice how stresses shape line 8's meaning as well as its rhythm. When the speaker mutters "Hell dry | you up | with its | flames," the trochaic meter stresses the word "you," connecting the speaker's curse to the roses that Brother Lawrence is so carefully watering: in essence, he's saying May you be as shriveled and burnt in Hell as your roses are well-watered on earth.