The speaker looks at men sleeping rough on the streets and listens to the sound they make together: a lament. Some of them curse, some pray, and the poem treats both as the same kind of cry. The lines about "hot words locked up" suggest anger and grief that have nowhere to go, and the poem offers one answer in a short line: "the key is kindness." It's a simple idea, stated plainly, as if the speaker believes it but can't quite act on it.
The ending turns the poem inward. Being near a corpse, the speaker admits to almost thinking "nothing of it." That's an uncomfortable confession. Seeing so much suffering has dulled the reaction a death should provoke, and the word "almost" shows the speaker is troubled by that dulling. The mood is quiet, sombre and guilty. There's pity here, but also honest shame about how easily people get used to what they ought not to get used to.