Actually, natural selection is far more cruel.
Had it been natural selection, the horse with weak legs would've broken its ankles in a meadow somewhere and lay there in agony until it died of starvation or was eaten by predators. Nature takes care of the weak and infirm by killing them early. Strength and intelligence are rewarded by breeding and passing those traits on to future generations. The weak are usually ripped apart and consumed in inhumane ways.
Eugenics tries to cut to the chase and eliminate the sloppiness of nature's brutal and drawn out process.