Sabrina le Fey is from one of the oldest and purest wizarding families- the family of Merlin. The long line of the Le Fey's are Slytherins. Merlin, Morgan, Mordred and Mortimer all were Slytherins in their time- Merlin being one of the earliest ones.
Sabrina wasn't always a cold-blooded killer. She went to school in Beauxbatons, pulling the occasional prank, and stealing an odd parchment of homework- she was not serious at all, and in fact was ditzy and very blonde (metaphorically, no offence). In school, she became close friends with the mysterious and alluring Carlisle Valois and they became pioneers of mindless French nonsense- or atleast, Sabrina was- for when they parted, Carlisle went on to carve a path in the arts of espionage and thievery, while Sabrina went back to England, to look for a job befitting the Le Fey name.
She went on to pursue the life of a Journalist, for the ghastly, controversial yet popular magazine, 'The Quibbler'; and landed a regular job there with a steady income. Her parents, Persephone and Mortimer le Fey had high-ranking Ministerial jobs in the Ministry of Magic along with her elder brother, Perseus. This made Sabrina and her younger brothers, Dunstan and Phoebus, privy to classified Ministry information. Sabrina, at the time, did not and could not care less, she was in her own happy bubble and was busy working on scoops concerning Nargle conspiracy theories for the next issue of the Quibbler.
Sabrina's bubble was soon to be burst abruptly. The pampered le Fey princess was about to get a reality check that would scar her for the rest of her life. Perseus, the eldest child, and heir to the le Fey name. Perseus had involved himself with dark wizards, and had turned a traitor to the Ministry. The group of Wizards had been targeting ancient families - destroying their lineage and looting their homes. The noble house of Merlin was their next target - and with Phoebus wrapped around their fingers, it was almost too easy.
It was Halloween- Sabrina was 19. The rain was lashing, and lightning illuminated the sky like candles lighting contorted faces carved onto pumpkins. They attacked in the dead of the night, killing Persephone first, then Mortimer. Both put up a fight, but they were caught off-guard, and were at a disadvantage. Perseus, who was dumbstruck at his 'friends' actions, tried to stop them and save his parents, but they tortured him and slowly killed him.
Sabrina and Phoebus were told to hide by Mortimer before the dark wizards entered, and they viewed the brutal murder of their family from the gaps of the large wicker laundry basket. Phoebus, only 10 years old, escaped from Sabrina's clutches and ran, crying to the body of his dead mother, only to be stunned then killed, too. But the wizards were not done yet- they looted the familial riches and trinkets, ornaments, money and anything of value. Sabrina, who was forced to witness her family's merciless and unnecessary death, grew cold and unforgiving. She notified Hogwarts, where her little brother, Dunstan, was studying (and thankfully out of harm's way) that Dunstan was the heir to the Le Fey name.
Dunstan was given leave from Hogwarts, for a while, needed to mourn the deaths, but he soon returned, and Sabrina was all alone- in a Manor house that had been stripped of it's riches and inhabitants. She swore to avenge her family's death- and would kill each of the wizards who had intruded in her house. Training in moving silently, brewing polyjuice potion, learning combative spells and also training in muggle swordplay, she trained and trained to attack and kill those who wronged her family name.
Alongside her training, she became active in the Grapevine. Her speciality- trading in the Black Market. Sabrina had a knack for procuring precious items for a select clientele in exchange for information. She dabbled in dark arts, and criminal offences- a bit of food poisoning here, extortion there- and finally was able to track down the organisation where her family's killers were.
She was ruthless and unforgiving. Using a sword and her wand, she killed every last one of them, and retrieved her lost heirlooms. Sabrina has remained this same person until date.