It's not difficult to understand. Terrorist attacks are the responsibility of the terrorists. Radicalisation is the responsibility of those dripping the honeyed lies into the ears of the gullible listeners.
That's all. There's no way to wriggle off the hook and say "I blew up a bus-full of children because my mother didn't love me" or "because a policeman didn't shoot me in time" or "because someone invented encryp;tion" or "because MI5 didn't see the message in the Times personal column"
Causation is not the same as responsibility. Responsibility requires conscious volition.
Yes, there may be a thousand factors, big and small, contributing to cause an event, but that doesn't make Twitter responsible for Bataclan.