Sherlock lay on the ground, blood pooling beneath him. He knew he didn't have long. How could he have been so blind? How did he not see that Mary Elizabeth Morstan was a cold blooded killer? How could he have let John get into such trouble? He had to tell him somehow. But how?
Blood. That's how.
Each movement was a struggle, but Sherlock managed to write one final message.
'Run, John. It was Mary.'