No one can say that the attack was Trump's fault alone.

But since entering the political sphere, he has called neo-nazis "fine people", called reporters "enemies of the people", called a congressman who assaulted a reporter "my kind of guy", offered to pay legal fees for his supporters who "knocked the crap" out of protesters, shared cgi videos of himself punching and hitting his opponents with golf balls, threatened that "second amendment people" would take matters into their own hands if he lost the election, legitimised hate publications like infowars, retweeted far right anti-muslim videos, said the previous president had founded the world's most famous terror group and seen a good deal of the people he has concentrated his attacks on receive bombs in the mail.

So I think when we see supporters from his side engaging in violent acts, we can hold him a little bit responsible.