Social media offers an avenue for like-minded individuals to interact in ways that were previously not possible. Yet, it can also be a breeding ground for hate and extremism to spread. Despite research into hate speech on social media, its influence on users adopting extremist beliefs is less understood. In this study we use causal analysis to quantify extremist adoption resulting from users becoming active in hateful online communities known as \textit{subreddits}. Using an Interrupted Time Series research design, we compare users who became involved in hateful subreddits (treatment group) to those who did not (control group). This analysis is reproduced across ten different subreddits covering four different topics: Alt-Right, Racism, Sexism, Fat-Shaming. From these analyses we uncover a causal link between a user becoming active in a hateful community and using more hate speech both within hateful subreddits and across the wider platform. The results are consistent and replicate across communities. Our findings are tentative evidence that users adopt extremist ideas from exposure to hateful subreddits.