The poem's central idea is a strange trade: when the speaker smiles at cruelty, something innocent dies. The title is oddly worded ("smile to cruel"), but the meaning seems clear enough. The speaker responds to something cruel with a smile, perhaps to be polite, to keep the peace, or to hide hurt, and that small act of giving in has a cost somewhere else.
The bird is the key image. It stands for something fragile and harmless, and its death happens "somewhere away," so the speaker can't stop it or even see it happen. Then the poem shifts. The dead thing lies "on floor in front of my eyes," so the distance disappears. The speaker now has to face what the smile caused, and calls it "the sacrifice of my weakness."
The feeling is guilt mixed with sorrow. The tone is quiet and bare, and the short lines leave a sense of stillness, like someone staring at something they can't undo. The double period at the end lets the thought trail off unresolved.