I took a few liberties with the following things:
Usage of Imperius-curse on house-elves
Order of employment of Heads of Houses
And last but not least qualities of the Slytherin Heads of House

Not too much... but still a bit...

There is a rule at Hogwarts

As a rule the potential Heads of House of Hogwarts got a mentor. Said mentor was originally decided by the former House of said person. Meaning: Salazars portrait teached the future Head of his House, Helga the new family-head of the badgers, Godric looked after the brash and Rowena was happy to hold the future Rawenclaw-Head responsible.

All went well for the first few years… ok… it might have been one to two hundred years… until Salazar had had enough… in a 15-years-timespan he had had to mentor seven new Heads of House, while the others had no one.

So, when the newest imbecile of Potions Master found himself on the wrong end of a unicorns horn, it was to no surprise to the Slytherin-Founder. (and what had that man, a three-timing, forty-years-old, MARRIED man been thinking trying to get his potions ingredients from the alive and literally KICKING source Salazar would never understand… in the end it had been the horn of the unicorn which had pierced the mans gut, but the animal had still got a few good kicks in beforehand. Of course such ingredients were always exponentially more powerful, and it was quite ambitious to try and brew an especially strong potion (and then to sell it).

That was still no reason to forget the self-preservation he tried to instill in his Heads of House. Of course it were always the muggleborns who found a fast end, because of their lack of understanding of the dangers of the wizarding world, which for a wizardborn were mostly common sense, something Salazar had been whining about for the last fifty or so years…)

So when Salazar found out that the new appointed Head was to be a muggleborn yet again, he hastened into the main portrait which he shared with his three friends and nearly yelled: "NO! No no no, I'll tell you a hundred times no if I have to… but I won't teach a muggleborn, Slytherin-Head of House again… not for another 500 hundred years! I will again, when they finally start to listen and stop being so obnoxious by refusing to learn self-preservation! I mean… it already started with a Bang! Who heard of a teacher being flamed in front of a class because he thought it ambitious to be the first teacher to show a class a living, FIRE-BREATHING Dragon!

"Then that idiotic woman who tried to proof that she could be plenty cunning by forcing the house-elves, without their immediate knowledge, to cook with shiny stuff, completely ignoring, that their minds are not the same as ours and that telling them to cook so that the food shines in Slytherin colours, while holding them under Imperius, lets them forget that metals are not edible for human consumation thus leaving them to use silver while said teacher herself is a werewolf … I mean… you remember the problems we had getting the elves to cook again because they were frightened to once again accidentially poisoning a member of the school body… We had to close off the kitchens…. Not to speak of the adjustments to the bewitchment that the elves have now, so they can't be used like this again…

"And lets not even to talk about the guy who was so traumatized by that devil's snare that he swore to never again step foot onto the school grounds… completely forgetting that it was his own fault for making the thing fire-resistent and then letting his lover enter the greenhouses with a lust potion. Imbeciles! Both of them…. Muggleborn imbeciles! Not even remembering that most potions that work on humans will also work on sentinent and half-sentinent animals and plants too!

"Of course not to forget the other four… the one who thought spying is a hobby and then forgot her polyjuice potion while being bedded by the king, who was decidedly NOT amused… the excecution was not all that swift as I recall…

"Then the … lady who wanted to show that even different runic languages can be combined and then completely forgot to check their meanings…. Did they find her yet, do you know? Well… it matters not…

"After that the one who didn't think a shield charm would be enough to save his students from an oncoming potions explosion, and jumped in front of them, completely forgetting the shield charm altogether.

"Still the king of them all is that moron who wanted to simulate a duel for his first years to proof that the levitation charm could be dangerous and then beheaded himself when he lost control over the sword he levitated…

"And now THIS… an idiot who didn't consider it important to remember that unicorns only let innocent females near them… just to win the favour of the king by brewing a doubly strong fertility potion for the kings wife.

"Seven… SEVEN ambitious, moderately cunning people who again and again failed to employ their self-preservation because they don't know how to learn common sense. Enough is enough… I refuse to teach an eighth!", he looked at Godric: "Maybe you can teach them how to be recklessly ambitious and still come out alive. That should be right up your forte."

And so the rule changed… since the next head of house who had been needed after that had been a Gryffindor who came from poverty and still scorned learning, it was Rowena who tried to teach him the value of books and knowledge to back up his brash students who were all to fast in getting into conflicts which could only be defused with the knowledge of obscure laws and similar things.

After that came a Ravenclaw who was all too proudly fighting with his family, similar to Rowena and her daughter Helena, so it became Helga who mentored him, trying to instill loyality and tolerance.

Next was a Hufflepuff who was too nice for even the nicest of the nice so Salazar took it upon himself to instill a bit of ruthlessness into her, all the while ignoring the rumors which had spread because of his cursing of muggleborn teachers, since he was still peeved about them and their famous lack of common sense.

When the next Head of House was someone who needed more courage than anything else, the founders started, without even meaning to, a tradition of rotation. And however strange it seemed to the new Heads, they didn't stop. It was by a strange twist of fate, that no founder mentored their own Head of House anymore…

For nearly 800 years that was the case…

Horace Slughorn was mentored by Helga Hufflepuff and furthered his ambition by being as friendly and understanding towards his students as she taught him to be.

The next new Head of House, Albus Dumbledore of Gryffindor, was taught by Salazar Slytherin to be cunning enough to refuse the ministers post to further his goals and rather influenced his students into accepting differences again by becoming the Headmaster of Hogwarts...

After him came Pomona Sprout, who, while as nice and friendly as befitting of the Hufflepuffs Head, learned from Godric Gryffindor how to be firm with others, even when it was just a more dangerous plant.

Minerva McGonagall was quite the prankster as a student but her mentorship with Rowena Ravenclaw taught her to be strict and teach her students how to search for knowledge to apply in their reckless endeavours.

Then of course came Filius Flitwick who learned from Helga Hufflepuff, that even his difficult circumstances as the son of a goblin and a human shouldn't hinder him in becoming the best Head of House he could be for his students.

And then this rule…even with this ‚new' tradition led to the first Head of Slytherin House since 800 years ago, who was not only a mostly muggle-raised half-blood, but also the first spy and Slytherin to be mentored by Salazar Slytherin himself.

And since Severus already was as brave as a Gryffindor with the right amount of resourcefulness, as loyal as a Hufflepuff, never shying hard work and as clever as any Ravenclaw with a right thirst of knowledge, Salazar searched what the boy was missing to be the most successful spy he could be and found it to be stealth and ruthlessness, but neither self-preservation nor common sense.

And so Salazar Slytherin took it as the Will of the Fates and agreed and taught Severus Snape.

Finite Incantatem

A/N: Which was your favourite 'idiotic' death? Could you agree with my reasoning? Please excuse any mistakes^^, English is not my mother tongue. Still I hope you liked it. Maybe leave a comment behind?

Greetings Phibby ^.~