It's never mentioned in the game, but Metal Gear Solid 5: The Phantom Pain has a hidden 'karma' system that, over time, alters Venom Snake's appearance. Your Demon Level is determined by your play style, with negative actions—killing people, harming animals, developing nuclear weapons, generally being a jerk—earning you Demon Points, which increase the size of the shrapnel 'horn' on Snake's head.
There are three Demon levels. There's the regular horn (pictured) that Snake starts out with. Then there's a longer one that grows in a cut-scene after earning 20,000 Demon Points. Then, if you manage to reach 50,000 Demon Points, the horn will grow even longer, and Snake will be permanently covered in blood.
Don't worry, though: you can lower your Demon level through positive actions like extracting animals, earning certain achievements, extracting child soldiers, or visiting Mother Base's zoo. Do enough nice things and Snake's horn will shrink again, and the blood will be washed away. You can raise or lower your Demon level as many times as you like, but it's purely aesthetic. You can't stab people with the horn, sadly.
For a comprehensive list of positive and negative Demon Point actions, and more details about the system, check out AlxCj's guide on GameFAQs.