Mumsnet has always been like this IME. There are just a lot of posters on here so you are bound to get some absolute assholes. I genuinely do think they are the minority though.
My tip is don't get heavily involved in any long threads (except really lighthearted ones maybe). The problem with the long ones is that people only read the most recent posts or all the op's posts at best and then they get all offended at something taken out of context and there's no talking people down once they've decided someone has been offensive, especially if someone else has already decided to be offended...
Bit of a ramble, but that's my analysis after quite a few years on MN.
If you want to comment just say your piece and move on. Don't get into defending yourself or what you've said because it is like trying to have a conversation with a brick wall once one or two people have decided you've done wrong and then others join in because it must be a good idea if everyone else seems to be doing it. It's classic mob mentality.
But again, this is the minority of threads and generally just the long ones on more serious / emotive subjects.
I avoid any long threads on bfing, climate change, step-parenting and child free by choice. Too emotive. Usually end up as nasty bunfights unfortunately.
It's a shame as there is value in the site imo, but I agree with you that there are some nasty bastards on here.
I'm on Reddit as well and other SM and I'd say you honestly do get it in lots of sites. It isn't just Mumsnet