Folk are deemed responsible for their own actions. (Even if in a deterministic universe this is questionable, but society can not exist in any other form.)
Whilst I believe those who think all Muslims are much the same and hold a desire to obliterate others, are wrong, expressing their puzzling opinion is no justification for the perpetrators of the attack.
Therefore expressed opinions bear no responsibility. It's not like those who expressed them were preaching violence against Muslims and encouraging the attacks.
The search for truth, and the debate that inevitably is part of that, must be considered unrelated to the actions of fanatics; as otherwise we would be put off even trying to understand anything just in case it can be related to something we'd wish to avoid. Such a head in the sand attitude would be to no one's benefit.