So, I might be anthropomorphizing a website and drawing too many parallels to my own experience, but having shifted my own stances here over the past 7-8 years, I can't say I blame Reddit.
I used to be completely against de-platforming people. And I still think sometimes it goes too far - people should be exposed to diverse ideas, and disagreeing with someone doesn't instantly make you a nazi. I won't mention names here, but there are a lot of people that I've vehemently disagreed with on major issues, but still followed along with 'sunlight is the best disinfectant.' I thought that they were ultimately exposing themselves as bigots and hatemongers when going on TV shows, writing op eds, showing up at college campuses. That they would end up taking care of themselves.
But then I realized that I was seeing more and more people quote them, quote their rhetoric. And I'm not even talking about stuff along the lines of 'Black Lives Matter' vs 'Blue/All Lives Matter' type stuff - people being blatantly anti-semetic, people glorifying pedophilia because "it happened to them and they turned out OK", all sorts of very cut and dry "bad things."
When we give these large public platforms to people peddling this, we're directly increasing the spread of the rhetoric. Reddit doesn't want to be involved in spreading this sort of rhetoric, even in it's milder forms, because people get radicalized slowly - you start with the softer sound bites, the ones where they have a lot more wiggle room to claim you don't mean this other thing. And as people start to agree with that idea, you move in with one that pushes the boundaries a bit farther. Keep repeating this process, and someone ends up embedded in the extreme forms of hate.
Reddit wants to cut that off at the start and not play a part in spreading these ideologies and helping radicalize people into them. They've realized that sunlight isn't enough to disinfect things that are fundamentally rotten, especially when these subreddits are largely monocultures without outside influence coming in. If they don't want to be a party to the radicalization of people into buying in on extremist hateful ideologies, they have to do this, and they can't stand by doing nothing in the name of "free speech"
On most of the internet, you don't win arguments by being right. You don't convince people by providing a well research, well sourced, informative argument. You win by beating people into submission. You win by having the better sound bites, the pithier quotes, the most upvotes and most people agreeing with you. And since these subreddits are so self selecting, when you wander in as an outsider, you only see the "winners" - the dissenting voices got downvoted, the bad arguments that appeal to the population of the reddit get upvoted.
Is this a lot of responsibility to put in the hands of people? Yes. Can it lead to the same sort of problem I'm describing here? Yes. Is there any other choice? Not that I can see. You can stand by and let something that you know is harmful and dangerous to human lives happen on your watch because you're afraid you might someday end up doing the same thing, and know that there will be bad results. Or you can risk it, try to take steps to prevent yourself from falling into the same trap, and move forward with something that is necessary with knowledge that if you are not careful you can become part of the problem.