A person can be sound and rational but people are tribal. It is deep human drive to find a tribe to belong to. What the internet has enabled is forming tribes irrespective of geographic proximity. Sites like 4chan/8chan allow people of radical persuasions to find each other and band together, growing their
That creates a powder keg waiting on a fight/flight trigger. Without a tribe to back someone, such a trigger will cause a flight response, but with that tribe they’ll feel emboldened and choose a fight strategy. Sites that encourage the formation of communities that promote violence will inevitably lead to actual violence once they’re past a certain size. It doesn’t even really matter what the ideology is, it just matters that they preach violence.