From Such Are Nightmares Made

The laugh, the bone-chillingly insane and evil laugh permeated every corner of the Doctor's unconscious mind.  He knew that laugh only too well, for it was his own warped future self, cruel and unyieldingly villainous.
The Doctor realized with terrible fear that he was attached with some irresistible yet invisible force to what appeared to be a seven layered target…
"Hello, Doctor…"
"What do you want?"
"You."
"I trust you mean that in an egocentric, completely insane way."
"Oh, Doctor, I want only your attention, and as for my egocentricity, it was developed from the culmination of the arrogance of several of my past selves, you not least of all…"  The voice floated around as if it came from everywhere and nowhere.
"So get on with it.  I've got places to be."
The laugh swam through the air as if it owned the gaseous mixture.  A spotlight suddenly shone on the Doctor, making him blink as his eyes adjusted.  And there, in the distance, on a platform floating in the sea of darkness, the megalomaniac stood, steely glare injected with tyrannical mischief.  "Doctor, how little you are prepared.  I am going to show you something you won't like."
"What exactly might that be?"
"Me.  Specifically my birth-your soul."
"I doubt you could have anything called a soul.  Your hearts are blacker than Omega's star."
"And where, pray tell, did I earn that evil?"  He laughed as the Doctor floundered for words.  "So I've silenced you with the truth!  How fascinating…"
"Oh, continue already!  At the rate this is going, I'll have reached the millennia mark before we begin!"
"Arrogant fool!  You see yourself as having never done wrong, the purity in your hearts unfettered by the muck you immerse yourself in to repair the evils of creation!  But you are wrong."
The sound of a staff striking a metal floor echoed through the poorly lit chamber.  The mood changed as if a switch had been flipped, the whole atmosphere growing chillier.  The center of the target onto which the Doctor was fixed lit up, stimulating a very old part of the Doctor's mind.
"What are you doing?"
"Seven circles, seven layers of Time Lord consciousness.  We are going to peel them away in study of what makes me-and therefore you-tick."
"I think you've done your maths wrong-I'm only in my sixth body, you know."
"I am aware of that.  It will be explained."  A light flipped on as the Doctor's first self was put on display.  "Exhibit A, as the humans say.  From him I gained irascibility and even cruelty.  I, however, would not have played such childish games as hiding the fluid link to investigate a city.  Though I must commend us on our ingenuity and blatant refusal to permit such a backward species as humans to dictate my course of action!  Let us dispense with him, shall we?"  Searing pain spread through the Doctor's mind as a piece of him was tortured into mindlessness.
"Ah, your-our-second self.  Incorrigible meddler, as I seem to remember.  While it is certainly true that such a quality has resided in each and every one of your-our-incarnations, this one contributed most.  Also his conniving blundering added a touch of something a little more…Prydonian, shall we say.  He had the foolishness to be caught in the act, however.  And for that, he deserves oblivion."  Pain stronger than the first time pierced the Doctor's mind as another bit of him was wiped away.
"Now we come to your pitiful third self."
"Our third self," interjected the Doctor.
"Yes, if you must remind me of that.  It was bad enough stooping to the level of actually helping those semi-evolved simians, let alone being so close to…Jo…" He said her name quietly as if it were a swear-word and he stood in a church.  "But you had to fiddle with their politics?  Preventing wars and chaos, oh dear the Time Lords won't like that."
"So what of that me contributed to you?  I don't seem to remember being particularly cruel or otherwise evil."
"Oh, but you weren't.  The fact contributing to my personality is obvious, Doctor.  You were a thief."
"Rubbish."
"You're conveniently forgetting that you-"
"We."
"-stole the Master's dematerialization circuit and found glee in the fact that he was then trapped on that most pusillanimous of planets.  Shall we…erase him as well?"  Once again, the pain in the Doctor's mind increased and he grew physically weaker as half of his previous minds were sitting in a state of complete ignorance to the universe around them.
"You've always been an ostentatious fellow."
"We," corrected the Doctor weakly.
"Must you keep reminding me of that fact?!  It grows tiresome.  I have a temper not unrelated to that of your fourth incarnation, I trust you'll remember that!  Now since I've told you of that quality, I can nullify him!"  This time the Doctor screamed, and a small sadistic smile crept over the tormentor's face as he watched his previous self suffer.
"Your fifth self gave me patience, despite his trust and belief that people are inherently good.  It is that patience which allows me to so successfully plan in advance-a trait none of my previous selves possessed."  Once again, the staff hit the ground.  Once again, the unavoidable searing pain swept over the Doctor's mind.
"And now we come to you.  With your regeneration on Androzani Minor, I had already gained a foothold into your existence.  Do you remember your attack on Miss Brown?  No?  Well, let us refresh your memory."  A vidscreen descended, lit up with the terrible scene as the Doctor cast paranoid glances at his companion.
"A peri is a good and beautiful fairy from Persian mythology.  The interesting thing is, before it was good, it was evil.  Just like you."  He lunged at his screaming companion, intent on only one thing-snuffing out the spy in his midst.  His hands closed around her throat as a primal and gleeful smile formed his face into a shape it was never meant to be…
"I…did…that?" croaked the Doctor from the table.  The laugh in the semidark came quietly.  
"Yes and no.  It was I who took her life into our hands.  I never liked that sniveling brat-her English was atrocious, she was an incessant whiner, and she never had the backbone to do anything halfway audacious!  The interesting thing is, your verbosity also crept into my being, as did your…"
"Arrogance?"
"Self-pride.  And if you do not cease your prattling, I'll be…forced to undo you."
"Not that you'll object."
"Of course I won't.  But if you're naught but a wisp in the ether, it would be rather complicated to gloat over what you are about to become."
"What?"
"The seventh layer is your next immediate self, bubbling away in your mind, fermenting, growing, preparing itself for life as you lay here in the midst of regeneration.  Oh, yes, Doctor, you're dying once again.  You didn't even have the decency to die for a purpose.  You're dying a pointless death.  But your next self is born for my purpose.  Oh, yes, at first your childish tendencies will cling to his conscience, but before long my agenda will be foremost in his mind.  Not that he'll realize who it is pulling his strings, I'm far too clever for that, but he'll do my bidding."
"Except I'll remember.  I'll stop you as I've stopped all the evils I've faced."
"No, you won't remember, and you can't stop me.  I am your antiself-a version of you which you cannot exist without, and yet cannot live with yourself for being.  And the poetry is that all your future selves will contribute.  The darkness of your ninth, the angst of your tenth, and so forth-"
"Not my eighth?"
"His…tenacity…will become my own.  But that is a fact you will learn for yourself without my interference.  Farewell, Doctor.  Pleasant regeneration," he said as he flipped the final switch, doing away with his sixth mind.  The last thing the Doctor registered before the regenerative energies faded was his own future self's evil laugh, the most terrifying evil laugh the Doctor would ever hear.