Quote 16
The excitement of the elm trees rising and falling, rising and falling with all their leaves alight and the colour thinning and thickening from blue to the green of a hollow wave, like plumes on horses' heads, feathers on ladies', so proudly they rose and fell, so superbly, would have sent him mad. But he would not go mad. He would shut his eyes; he would see no more. (1.62)
Having experienced sheer terror, Septimus is really moved by visions of beauty (heck, we’re moved by beauty even without this terror business). The trees are very suggestive to him, just as flowers are suggestive to Clarissa.
Quote 17
Men must not cut down trees. There is a God. (He noted such revelations on the backs of envelopes.) Change the world. No one kills from hatred. Make it known (he wrote it down). He waited. He listened. A sparrow perched on the railing opposite chirped Septimus, Septimus, four or five times over and went on, drawing its notes out, to sing freshly and piercingly in Greek words how there is no crime and, joined by another sparrow, they sang in voices prolonged and piercing in Greek words, from trees in the meadow of life beyond a river where the dead walk, how there is no death. (1.70)
Septimus dwells on the idea of the crime, which is never totally defined. Woolf seems to suggest that the crime is beyond one person’s actions.
Quote 18
He said people were talking behind the bedroom walls. Mrs Filmer thought it odd. He saw things too – he had seen an old woman's head in the middle of a fern. (4.37)
Septimus’ shell-shock involves seeing and hearing strange things. It’s hard to avoid the judgment of others.