How would I change the evolution of humans to result in horns on humans as a result of reproductive fitness, instead of social fitness. Meaning, NEITHER "Horns are sexy", or some strange infection.
A predatory creature with claws and large teeth is unlikely to evolve horns. Unlike herbivores, carnivores have evolved to kill things, and don't tend to need to evolve extra weapons like horns. Furthermore, effectively using horns would require that the creature put energy into growing them, and into growing and sustaining the supporting structure needed to make them useful. Horned creatures need strong necks to support use of their horns, and the neck musculature wouldn't be critical to anything else for a creature evolved to run down and claw up its prey.
However, Hair is Keratin, Nails are Keratin, could hair evolve into horns?