Hate speech is a growing concern online, with minorities and vulnerable groups increasingly targeted with extreme denigration and hostility. Why users express hate speech on social media is unclear. This study explores how this hate speech develops on both mainstream and fringe social media platforms; Facebook and Gab. We investigate whether users seek out hostile areas of these platforms in order to express hate, or whether users develop these opinions through a mechanism of socialisation, as they interact with others over time. We find evidence that some users do arrive on these platforms with pre-existing hate stances, while others develop them with time spent on the platform. We find that hate speech is unevenly distributed, with a small number of users contributing a large proportion of the hate on the platforms. Our analysis reveals how hate speech develops online, the important role of the group environment in accelerating its development and gives insight into the development of counter measures.