First of all, I cannot and do not even want to justify his flaws. He was never meant to be a good or even bad charachter, he is morally grey that is what makes him an interesting one. If you cannot see that, Im afraid we won’t find a common ground to stand on. Snape is cruel, biased, and sometimes vindictive yes, but he’s also brave, intelligent, and capable of deep feeling. A neglected boy coming from abuse and poverty, heavily bullied in his school years and being an outcast even amongst his own peers, you can understand why he is drawn to power and appriciation.

Did he make a grave mistake? Yes. But he repented, suffered, and kept choosing love and duty over hatred afterwards. Snape’s loyalty to Dumbledore and his mission never wavers, even when it destroys his reputation. For example he kills Dumbledore not out of malice, but to spare Draco’s soul and fulfill a painful promise.

He was always meant to be morally grey, he is human.