If I had to pin down one synonym for 'heartless' that reads like a line of poetry, I'd choose 'ruthless.' It has a cold kind of music—hard consonants that snap, but it also carries an implied method, a clarity of purpose that feels almost classical. When I say 'ruthless' in my head, I see a winter coastline: bare branches, wind that knows no compromise. That imagery is useful in verse because it lets the reader feel a deliberate cruelty rather than random emptiness.
I also like how 'ruthless' can sit beside literary references without collapsing under melodrama. Put it next to a clipped allusion to 'Heart of Darkness' or a stark scene from a modern novel and it expands, suggesting not just lack of feeling but a philosophy of action. For my taste, that layered meaning gives a line weight and opens room for metaphor, so I often reach for 'ruthless' when I want a word that stings but still sings in a poem. It always leaves me with a slightly bitter, satisfied aftertaste.