When what your dealing with or being made to do is so unbearable or boring that you would rather shoot yourself in the head and have your brains and blood splatter all over the wall. Thus "painting" it.
When someone mainly in prison anally rapes some one and gets poop and blood all over its penis so they smear their penis on the wall like a paint brush know as paint the wall.
Don't let me catch you alone or else I'ma paint the wall with your ass