Why is Snake covered in blood even after a shower?

From what I've read here, the only way to get the blood off of snake, is by lowering your "Demon level".

10- Can I shower off Demon Snake's blood?

No, it can only be removed by lowering your Demon Level.

You can lower your Demon level via the following actions:

DPG06-----Losing Demon Points-----

The following is a list of actions the players can perform to decrease their Demon Points, grouped by category:

---Fulton extractions

-30 - Fulton extract a common animal
-30 - Fulton extract a critically wounded enemy soldier
-30 - Fulton extract a puppet soldier via helicopter
-30 - Fulton extract a critically wounded enemy soldier
-30 - Fulton extract a Skull
-60 - Fulton extract a rare animal
-60 - Fulton extract a Diamond Dogs staff member
-60 - Fulton extract a prisoner
-120 - Fulton extract a child
-120 - Fulton extract a very rare or extremely rare animal

---Extractions by helicopter

-30 - Extract an enemy soldier via helicopter
-60 - Extract a puppet soldier via helicopter
-60 - Extract a critically wounded enemy via helicopter
-60 - Extract Quiet via helicopter
-120 - Extract a VIP or prisoner via helicopter
-120 - Extract a Diamond Dogs staff member via helicopter
-240 - Extract a child via helicopter

---Trophies

-1,000 - Earn the "Cry Havoc" trophy
-1,000 - Earn the "Speechless" trophy
-5,000 - Earn the "Cleared" trophy
-5,000 - Earn the "Trusty Steed" trophy
-5,000 - Earn the "Man's Best Friend" trophy
-5,000 - Earn the "War Buddy" trophy
-5,000 - Earn the "Reminiscence" trophy
-10,000 - Earn the "Conservation" trophy

---Other

-300 - Visit the Animal Conservation Platform
-1,000 - Dispose of a nuclear weapon
-30,000 - Dispose of ten nuclear weapons

And here are the actions that can raise your Demon level:

DPG05-----Earning Demon Points-----

The following is a list of actions the players can perform to increase their Demon Points, grouped by category:

---Fulton extractions

+20 - Fail the fulton extraction of an animal
+60 - Fail the fulton extraction of an enemy soldier

---Assassinations

+20 - Kill an animal
+60 - Kill an enemy soldier
+90 - Kill a Diamond Dogs staff member held hostage
+100 - Kill a prisoner
+120 - Kill a critically wounded enemy soldier
+180 - Kill a Diamond Dogs staff member

---Assassinations via fire damage

+40 - Kill an animal with fire damage
+120 - Kill an enemy soldier with fire damage
+180 - Kill one of your Diamond Dogs staff members held hostage with fire damage
+200 - Kill a prisoner with fire damage

---Buddies

+75 - Destroy D-Walker
+150 - Critically wound D-Dog, D-Horse or Quiet

---Other

+30 - Critically wound an enemy soldier
+60 - Lose a staff member held hostage by aborting mission 22 (+60 each)
+150 - Destroy a support helicopter
+50,000 - Develop a nuclear weapon

There is a hidden stat in the game - Demon points, which are basically the opposite of heroism. Demon points are gained in various ways - Killing animals, enemy soldiers, prisoners etc.

These points accumulate behind the scenes and are not viewable. When your demon points get too high you will see changes such as being permenantly covered in blood and the horn on your head growing in size.

Demon points can be reduced by doing things like extracting animals, children and enemy soldiers via helicopter.

Here is a pretty in depth guide to explain things in more detail: http://www.gamefaqs.com/boards/718564-metal-gear-solid-v-the-phantom-pain/72466130