So, there’s no video of him hitting Heyer?
All I can say is that I searched for such a video around 8-27-17 and never found one, though one may exist. News reports said that Fields plowed into pedestrians, hitting Heyer and others, then backed up. That is consistent with the video we saw. (By the way, that video was hosted on the Daily Caller website, but is no longer there.)
Fields's attorney could have made an excellent case that Fields had acted in self-defense if the crowd had spontaneously attacked the Challenger, and Heyer had been hit during Fields's retreat, rather than when he was going forward. That might have reduced the charge to manslaughter. From what I could tell from news reports, the lawyer did not make that case. That plus Fields's guilty plea tells me that he and his attorney decided that they would be unable to justify Fields's actions to a jury, which in turn means that he hit Heyer while he was moving forward.
I wanted to hear Fields explain in his own words what had happened. If he was not guilty, he should have wanted to tell his side of the story. He did not, which also points to his guilt.
On the other hand, he may have decided that the deck was so stacked against him that he had no chance of a fair trial, so he pleaded guilty just to avoid the death penalty. That seems possible to me, but not plausible.
I add it all up and arrive at "probably guilty."