The Daleks claimed to have done away with emotion, but that was a lie. They knew nothing of the gentler emotions, it was true, but pride and arrogance and superiority were part and parcel of their make up.
They considered themselves the acme of evolution, the highest and purest example of intelligent life that had ever existed, and as such, judged all lesser races to be an intolerable contamination of their purity, deserving only to be exterminated. Anything that wasn’t a Dalek was by definition inferior, a waste of resources.
They would not stop until all other life was destroyed.