The very first thing Bellatrix wrote, in her very first letter home, was a confirmation that she had been put in Slytherin. Her parents, overjoyed, wrote back in haste to counsel her on the joys of the house. Slytherin, they told her, valued all virtues – loyalty, wisdom and bravery – but above all it put stock in ambition and good sense. Slytherin's reputation might be unfavourable with the other houses, but her parents told her, it would be in Slytherin that she would find her true friends. Unsurprisingly, the letter also reminded her that in Slytherin house, the Black family reputation would proceed her and it would be more important than ever to live up to expectations.
Whether that was too great a burden for a child of eleven was a question no one thought to ask.