It can be common in certain fanfiction circles. Some fans might write this to subvert the traditional narrative. In the original 'Harry Potter' story, Snape is a morally ambiguous character. By having Harry kill Snape in fanfiction, it creates a new power dynamic and a lot of room for exploring different emotional responses from other characters like Hermione and Ron, as well as the overall impact on Hogwarts.
Yes, it is a somewhat common theme in some fanfictions. One reason could be the complex relationship between Harry and Snape in the original series. Snape was often cruel to Harry, so in some alternate universes, writers explore the extreme scenario of Harry killing Snape. It also allows for exploring darker aspects of Harry's character and the consequences of such an act within the wizarding world.