I'll hold back until more of the attacker is known. However, unless he had some kind of identifiable mental illness, I would say he was very much of a sound mind. Otherwise every single murderer, including ISIS fighters, could be described as not being of sound mind.
If this does turn out to be related to the referendum or the Syrian War, I hope some people will think about the hatred which has been unleashed. The referendum decision should be based on rational decisions, not the emotive hogwash and personal attacks, which has become part and parcel of the campaigning.
My thoughts are with Jo Cox and her family.