I think you're approaching this from a legal standpoint. I don't think there is enough there to convict him, legally, for causing a riot. I'm not making a legal argument against him.
He told his followers that the election was rigged. Is that illegal? No. In the middle of the chaos, Trump tweeted out disparaging remarks about Mike Pence which effectively threw gasoline on the fire that he started. Is that illegal? No.
Morally, I blame him for what happened that day, whether he's convicted of it or not. I expect better from the President of the United States. He's the only President we have ever had in our entire nation's history who did not allow a peaceful transition of power. I have an issue with that behavior, illegal or not. And I'm not the only one.