The narrator's character is revealed throughout the poem by his attitude towards Brother Lawrence.
Gr-r-r-there go, my heart's abhorrence!: Is rage and hatred an appropriate attitude of a monk towards another monk?
Water your damned flower-pots, do!: What about swearing at flower pots?
If hate killed men, Brother Lawrence,
God's blood, would not mine kill you!
Taking God's name in vain wishing to kill someone by force of hatred rather than turning the other cheek are both unchristian.
What? your myrtle-bush wants trimming?/ ... Hell dry you up with its flames! Again, why should a monk want someone to suffer in Hell merely for tending a garden?
Or, my scrofulous French novel,
On grey paper with blunt type!
Should a monk be reading pronography? And trying to tempt other monks with it?
As you read line by line through the poem, look closely at what the narrator is saying, taking into account that the narrator is a monk.
Robert Browning was brought up as a Dissenter, and in this poem is promulgating a Protestant view that the Roman Catholicism of the narrator encourages mere surface piety (crossing fork and knife) rather than a genuine charitable Christian spirit.