For me it is religions, and although we could cite so many other things that cause division and conflict, and which have resulted in so many deaths, religions are the main beliefs most difficult to alter or eradicate (in my view), mostly because they are based on material from the past that is essentially taken 'as is' (and not provable), even though we have so many different interpretations of such. For all the good they have done, we just don't know if our history would have been better and perhaps less troubled if they hadn't existed - or at least in the forms they took. But perhaps we were just destined for what we got no matter as to whatever such beliefs formed.
I suppose that ideologies, as to religions being a subset of these in my view, could be labelled as being the most dangerous thing though.