All he remembered was running. He was running away from the inferno that lay behind him. He could not quite recall why, but all he knew that happened next was a loud crashing sound, and the buckling of his knees. Obi-Wand Kenobi remembers that he took quite a blow to his head, and as he fell into a darkened state of consciousness, learned the Truth.
The Truth did not lie in the bowels of Hogwarts, nor did it lie in his once lifetime goal of becoming a St. Mungo's nurse. Obi-Wand Kenobi was destined for a future far greater than that of wizard. He saw himself as a proud Jedi knight, gallantly swinging his lightsaber under the name of justice.
Obi-Wand was so taken by this vision that he woke up in the hospital wing (many months later) screaming passionately that he would undertake his lifelong mission to defend and exercise justice in all realms of the galaxy. Madam Pomfrey was quite alarmed, believing that the newly awakened Obi-Wand had gone insane.
"Insanity?" said he, "What in the blazes is insanity! My friend, I sought to seek a greater beyond among the stars! Discharge me from this infirmary. I must go forth to protect the code of justice!"
Madam Pomfrey denied Obi-Wand a chance at freedom and insisted that he must stay in bed. Obi-Wand did not take this too kindly, and attempted to use Jedi mind tricks on Madam Pomfrey.
Initially, he believed that he would be successful in this, beaming a confident smile reminiscent of a certain Gilderoy Lockhart, and waving his hand in front of a bewildered Madam Pomfrey. "You shall let Obi-Wand leave," he said softly, "You shall let him defend the galaxy."
Madam Pomfrey remained adamant on confining him. "You are not to leave the hospital wing until you are well, mentally and physically," she said, "And if you try any more of this, I will have to subdue you by magic."
Then, it dawned on Obi-Wand. There must have been a reason among reasons that Madam Pomfrey had not been swayed by his mind tricks. Most importantly, she had denied him access to protecting justice.
Madam Pomfrey was a powerful Sith Lord. And she must be stopped.