It's a very difficult balance in my opinion.
You ban these people then you lose visibility of them yet if you leave them where they are you allow them to use it to indoctrinate others.
I agree these platforms should properly police their content and things like this should never be allowed to happen but you have to weigh up who then polices them and who decides what to remove?
Acts of a criminal nature should always and without question be banned and blocked before anyone sees them or has a chance to download or share themselves because acts of a criminal nature have been through the due process to define them.
I just hope there are no knee jerk reactions in all this and any actions taken are thought through properly first as sad as this act of terrorism is. What I don't want to see is government using it to extend their reach.