I'm not sure of a specific 'cruel Snape fanfiction' title off - hand. However, you can often find such stories on fanfiction websites. Just search for tags like 'cruel Snape' or 'Snape dark side'. Some fanfictions might show Snape being cruel in his relationship with other characters, for example, his interactions with Harry Potter could be made even more hostile than in the original books.
There are numerous 'cruel Snape fanfiction' out there. A lot of them might be set during Snape's time as a teacher at Hogwarts. In these, his usual cold and strict demeanor could be amped up to be truly cruel. For instance, he might give out harsher punishments, or be more unyielding in his grudges. Fanfiction writers often like to explore this side of Snape to add a different dimension to his character, but it's important to note that this is a departure from the complex yet ultimately more nuanced Snape in the official books.
One 'cruel Snape fanfiction' could be 'Snape's Dark Trials'. In this, Snape might be depicted in a harsher light, perhaps with his strict teaching methods turned into something more extreme for the plot. Another could be 'Snape's Cruel Intentions', which may explore his more complex and unforgiving side within the wizarding world setting. There are also some works where Snape's actions towards students are exaggerated in a cruel way, like 'The Cruel Snape at Hogwarts' which delves into how his presence affects the students in a more menacing manner.