I can picture the disenthralled spirit sitting by the lake at night staring off into it's wholeness and being happy with the peace that the darkness brings. It also seems to be questioning how well the person knows the soul. The last word of the poem is fear. Fear of what might be revealed if the two become as one. The soul lies buried within much like the deep, dark waters of the lake become eerie, unknown, and deathly cold the farther one goes down.
Fear is present in the mind, in the body.
Swinburne liked to be daring, different, and shocking. Philomela is a victim of rape, which is horrible. That is much more gruesome, plus no one else focuses on his side of the story. ACS was being different and shocking by choosing to write about Itylus. But in her eyes I guess she was doing the most revengeful thing she could think of.