Think about it
There were all kinds of people in London, tall and small ones, old and young ones. The population of the whole world together in one city.
Under those people was also one Severus Snape, a tall and thin man with black hair and a hooked nose. He wasn't often seen in Muggle London, as he called it.
The reason for that was easy: He was a wizard and therefore preferred to stay in the wizarding world.
But sometimes there was nothing better than a little time away from the people who knew him. Severus was a former Death Eater and most people despised him for that. There were only a few witches and wizards who didn't hate him for having chosen to serve Lord Voldemort and they were either followers of the Dark Lord themselves or they had the same opinion on Muggle-Borns as him but didn't want to fight.
The Muggles didn't know anything about the war that took place right in front of their noses and they just treated him like any other stranger. He could escape all the prejudice, the people who blamed him for the pain they went through and the pain he felt himself. It was relaxing to just walk through the city, drink a cup of coffee somewhere and watch the Muggles around him.
While Severus walked down one of the many streets, there was a blond woman in front of him. She was about fifty years old and seemed to be a teacher with her class. They were near enough for him to hear what they were talking about.
The students were discussing a book and the teacher gave them a few questions to answer. One of the topics was a mother who had left her husband and son.
The teacher wanted to know a good reason for a mother to leave her child behind. One of the girls answered, "I think it would be OK for the woman to go away if her husband her."
The woman agreed, "Yes, if my husband hit me, I'd stay away from him. That's a No-Go." She laughed but none of the students joined in on her laughter.
Did she really think that was funny? A girl with curly black hair looked particularly angry. She looked exactly like Severus felt right now.
How dare that woman make jokes about this? His own mother had been hit by her husband but she hadn't left because she had tried to protect her son. When Severus had been sixteen, his father had had a bad day and wanted to take it out on his only son. Severus' mother had prevented that but it had just made his father angrier.
So angry that he had killed her.
Severus had been there and had seen how his father had killed his mother for protecting him. It had changed his life forever and he still had nightmares about it so there really was nothing funny about that subject.
Without thinking about it, Severus quickened his pace and approached the class. He walked next to the teacher and said "There are many women in the situation you just made fun about. Some even die at the hands of their husbands. There are enough funny things you can make jokes about but this is not one of them. Think about it."
With that he turned into a little alley and disappeared in the shadows. He didn't notice the teacher and students stare after him, nor did he see the small smile one the black-haired girl's lips.
He had done his job. Severus knew he could never repay his mum but at least he could make sure her death wasn't in vain by getting other women who suffered through the same help and making ignorant people aware of the seriousness of the situation.