Symbolism, Imagery, Allegory
Singer's hands are pretty stinkin' impressive: they produce art, words, and deep human connection:
His hands shaped the words in a swift series of designs. His face was eager and his gray-green eyes sparkled brightly. With his thin, strong hands he told Antonapoulos all that had happened during the day. (1.1.4)
But after Antonapoulos gets taken away, Singer stops using his hands. In a way, he's withholding his beauty from the world: he doesn't even try to communicate with others. To make matters words, his hands continually taunt him, almost begging to be used:
His hands were a torment to him. They would not rest. They twitched in his sleep, and sometimes he awoke to find them shaping the words in his dreams before his face. He did not like to look at his hands or to think about them. (2.7.27)
When Singer stops using his hands, we can see the effects that a lack of human connection can have on a person. So it's rather fitting – if really dark – that for their final act, Singer's hands pull the trigger of a gun in an act of suicide.