Harry Potter is a cold blooded killer. He was raised to hate. He hides this from others well, but every so often he catches the eye of the head master or Snape.

He befriends Hermione and either Ron/Draco/Nevile/Blaise (boy)/Dean/ Theodore Nott/ any male charecter in the school, but the Trio must have Harrry / Hermione / male. He also comes to see Dumbledore as a mentor despite how diffrent their ideals were, they become close confidents in each other. Harry is still frieds with Hagrid due to him introducing him to a world of magic.

He forms a group of followers. In his second year he kills Gilderoy Lockhart. In his third he does not obtain the 'marudars map' because he is not a friend of Ron or any Weasley (Except if you are making one (or more) of them in Harry's group o friends / followers).

He finds a book in the chamber of secrets, I sugest behind Salazars mouth. It turns out to contain Magic so vile and disgusting that Salazar decided to bind it via a spell which Harry atempts to crack open over the following years, Slowly decoding page and learning spells. the Fiend Fire curse or how to make Inferi-, maybe the Hocrux ritual, those pages were already translated by the last person to find them, Lord Voldemort. however he graduated before he could crack it, this means that you cannot teach Harry spells quickly and that although Voldemort cracked it he also learned other things, you can not make him over power Voldemort with spells like ~ Explodus Brainium ~ and expect Voldemort not to know the counter spell. 

If you make Draco into one of the followers that Harry has, kill off Lucious and Narcissa. Harry kills them, he does not notice Draco who observes it. If you are to kill off Lucious and Narcissa, Draco MUST be his friend. Draco runs to the Headmaster and tells him of what he saw. If not Draco then Daphne Greengrass and her parents / Susan Bones and her aunt / Neville and his grandmother / whoever it is you make a follower of Harry and their close relative. 

Harry finds a page which is locked tighter than any of the others and decides to decode it. He figures out that it is a ritual to summon Death, A rope which hanged a man and a sword which has slain a woman are required for this. 

You can fill on the rest of Harrys years at Hogwarts apart from a few things.

No Weasley, Malfoy, Snape, Dumbledore, Ministry or any other kind of bashing.

Dumbledore must not be manipulative or senile in any way, he honestly thought that Harry would grow up happy if a bit neglected compared to Dudley. 

Harry must kill the Dusleys and several others, you must also show the consuqencas of this, if you kill Ron, don't make Molly laugh at a joke someone tells her five minutes later, perhaps have her pull her children out of school for a while, think about it that was a persons child! Show the family/friends of people Harry's killed and their grief. 

Despite Harry's confidence he leaves several paper trails and clue's.

Eventually Harry must fight Voldemort and win, at a cost, whether it be his followers, his friends or even a limb, but the price must be grave. He use's several Black Magic curses and spells in his duel. The Ministry of Magic steps in to do an actually smart thing and cull the Dark Lord in the making(Don't Villify them, perhaps several bystanders were killed or they find out about his killing, it's up to you). Dumbledore however refuses to let them do this and begins dueling the ministry aurors with Harry. They both flee together and stay on the run for a while were Harry confeses to Albus about his activitys, this can come to blows if you like, but don't split them uo. Albus can be disapointed in Harry or just be in shock. Albus may know some Dark arts if you like, due to Grindewald or otherwise.

You can go down several paths with this Time Travel which if you d It would preferably be to the past. 

You can send Albus and Harry to Azkaban if you do then have them plan to escape, don't have them wait out their prison sentance.

Make Harry and Albus teacher and Student while still on the run,

Anything you want really. 

Just to clarify here is the Challange and its perimaters

* Harry is still friends with Hermione but you can make the other member anyone, so long as they are male * 

* Harry kills the dursleys soon after returning from his shoping trip with Hagrid in book one, before this he has no knowledge of magic * 

* Harry forms a group of sorts in second year, he does this to protect the 'members' of the 'group', it is more of a lot of random people who are connected to Harry who become 'partnered up' for lack of a better term * 

* Harry kills Lockhart during second year, probably when he tries to perform obliviate on him would be a good time * 

* If Harry is NOT affiliated with the weasleys in any way, don't make him steal  the maruaders map or 'convince' the Weasley Twins to give him it *

* Harry finds a book in Hgwarts, in either His third or Fifth year, which is so powerfl Slytherin locked it away in his chamber with numerous charms on it, several pages are 'unlocked' such as the Fiend Fire, Inferi and Hocrux pages Harry atempts to breack more pages open with little sucess, decoding about a paragraph of writing a month, Voldemort 'Rediscovered' the unlocked ancient spells *

* Harry kills someone important to a person in his 'group' and they tell Dumbledore who dosen't believe it and brings it up with Harry in a conversation * 

*Harry discovers a heavily warded book in the Sealed 'Grimore' which he translates as a ritual to summon death (DO NOT have Harry do this) requiring a Rope which has hanged someone and a Sword which has slain a woman, it is optional for Voldemort know this and summon 'Death' to kill Harry, in that case make it a demon not death. * 

* Dumbledore is not manipulative or Senile and definatley dosen't rob orphans *

* When Harry kills someone show his reaction, maybe a tighting fist or several deep breaths * 

* Show the people Harry has killeds Family and frieds, just to show death isn't cheap *

* No Bashing EVER * 

* Don't use stupid nicknames like Moldyshorts or Smelly Warts, simply don't *

* Harry and Voldemort Duel and several bystanders are hit and killed due to the Black Magic Harry has discovered, the ministry stepin and atempt to lock Harry up, Dumbledore and Harry resist his capture * 

* They flle and go on the run for several weeks to several years, at which point yu should put in a huge plot point like they were captured! or Time Travel! * 

* Harry leaves fingerprints or footmarks or whatever, but several partys catch on, maybe Moiody, Snape or The Ministry * 

* for the most part try to stick to cannon, that means Harry isn't a Veela or a Hulking mass of flesh fifteen foot tall * 

* At the start of each new chapter place an obituary *

Ok what you do have control over, 

His house, don't change the other peoples Cannon house through. 

Who lives and who dies 

Most of the plot

And a lot of other stuff

Sincerly Lewis The King

On a difficulty level I would say 8 / 10 It would take about four thousand words to describe all of it in the most basic form whilst still keeping the plot and if you do this, you aren't going to do it half-hearted and come out with something good, if you choose to do it as a one shot try for about 10,000 + if its a story try for about sixty thousabd + at the least.