Yes, when you create a venue for free speech, it will -- gasp -- be used to express various opinions and ideas you will not agree with. So?
The Nazis had no trouble doing their dirty work the last time without Facebook, Reddit, or Twitter. These days, they have more to lose from exposure than they would ever have to gain. The antidote to bad speech is more speech, and that's precisely what Reddit is good for.
History does not support your naive view that allowing criminals to freely congregate, organize, and recruit stops their ideology.
The Neo-Nazis on reddit have publicized the tactics they're using to normalize racism across the internet. According to watchdog groups the Neo-Nazi movement has grown tremendously since it was able to start recruiting and spreading that hate ideology online.
The Nazis had no trouble doing their dirty work the last time without Facebook, Reddit, or Twitter. These days, they have more to lose from exposure than they would ever have to gain. The antidote to bad speech is more speech, and that's precisely what Reddit is good for.