The speaker looks back on a boy they once knew, and the poem slowly reveals that his "paintings" are not what we first expect. At the start he sounds like a lonely artist, working at night in a smoky bedroom where nobody can see his work. The details feel secretive and sad: he is alone, hidden away, and the images are "strange" and "twisted." Then the last two lines change everything. His brush is a razor and his canvas is a wrist, so the art is really self-harm.
The speaker seems gentle and sorrowful, perhaps haunted by the memory. The phrase "once upon a time" sounds like a fairy tale but turns bitter, since this story has no happy ending. Calling the harm "magnificent paintings" suggests how the speaker wishes they could have seen him, or how pain can be disguised as something beautiful. The mood is quiet, dark, and heavy, and the sudden reveal makes it hit hard.